[Foundation] Performance improvements for IndexPath bridging and comparison

[phausler]

The previous implementation of IndexPath would cause a malloc of the underlying array buffer upon bridging from ObjectiveC. This impacts graphical APIs (such as UICollectionView or AppKit equivalents) when calling delegation patterns. Since IndexPath itself can be a tagged pointer and most often just a pair of elements it can be represented as an enum of common cases. Those common cases of empty, single, or pair can be represented respectively as no associated value, a single Int, and a tuple of Ints. These cases will be exclusively stack allocations, which is markably faster than the allocating code-path. IndexPaths that have a count greater than 2 will still fall into the array storage case. As an added performance benefit, accessing count and subscripting is now faster by aproximately 30% due to more tightly coupled inlining potential under whole module optimizations. Accessing count is also faster since it has better cache-line effeciency (lesson learned: the branch predictor is more optimized than pointer indirection chasing).

Benchmarks performed on x86_64, arm and arm64 still pending results but should be applicable across the board.

Resolves the following issues:
https://bugs.swift.org/browse/SR-3655
https://bugs.swift.org/browse/SR-2769

Resolves the following radars:
rdar://problem/28207534
rdar://problem/28209456

This was re-opened since the CI server decided it didn't like the previous PR

[phausler]

@swift-ci please test

[phausler]

@swift-ci please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - 823c498
Test requested by - @phausler

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - 823c498
Test requested by - @phausler

[phausler]

StringIndex should not have any interaction with IndexPath

[ RUN      ] StringTests.ForeignIndexes/UnexpectedCrash (XFAIL: [Always(reason: <rdar://problem/18029290> String.Index caches the grapheme cluster size, but it is not always correct to use)])
stderr>>> fatal error: Can't form a Character from a String containing more than one extended grapheme cluster: file /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/swift/stdlib/public/core/Character.swift, line 183
stderr>>> Current stack trace:
stderr>>> 0    libswiftCore.dylib                 0x000000010ad0e930 reportNow(unsigned int, char const*) + 125
stderr>>> 1    libswiftCore.dylib                 0x000000010ad587b0 _swift_stdlib_reportFatalErrorInFile + 100
stderr>>> 2    libswiftCore.dylib                 0x000000010aa3b2a0 closure #1 in closure #1 in closure #1 in _assertionFailure(_:_:file:line:flags:) + 320
stderr>>> 3    libswiftCore.dylib                 0x000000010aa3b270 closure #1 in closure #1 in closure #1 in _fatalErrorMessage(_:_:file:line:flags:) + 35
stderr>>> 4    libswiftCore.dylib                 0x000000010acece70 partial apply for closure #1 in closure #1 in closure #1 in _fatalErrorMessage(_:_:file:line:flags:) + 97
stderr>>> 5    libswiftCore.dylib                 0x000000010aa37570 specialized StaticString.withUTF8Buffer<A>(_:) + 204
stderr>>> 6    libswiftCore.dylib                 0x000000010acecdb0 partial apply for closure #1 in closure #1 in _assertionFailure(_:_:file:line:flags:) + 146
stderr>>> 7    libswiftCore.dylib                 0x000000010acecd90 partial apply for closure #1 in closure #1 in _fatalErrorMessage(_:_:file:line:flags:) + 23
stderr>>> 8    libswiftCore.dylib                 0x000000010aa37570 specialized StaticString.withUTF8Buffer<A>(_:) + 204
stderr>>> 9    libswiftCore.dylib                 0x000000010ac56660 partial apply for closure #1 in _fatalErrorMessage(_:_:file:line:flags:) + 159
stderr>>> 10   libswiftCore.dylib                 0x000000010ac56600 partial apply for closure #1 in _fatalErrorMessage(_:_:file:line:flags:) + 23
stderr>>> 11   libswiftCore.dylib                 0x000000010aa37570 specialized StaticString.withUTF8Buffer<A>(_:) + 204
stderr>>> 12   libswiftCore.dylib                 0x000000010aa37470 _fatalErrorMessage(_:_:file:line:flags:) + 126
stderr>>> 13   libswiftCore.dylib                 0x000000010ac32930 specialized String.subscript.getter + 1523
stderr>>> 14   libswiftCore.dylib                 0x000000010ab25a40 String.subscript.getter + 9
stderr>>> 15   a.out                              0x000000010a991410 closure #9 in  + 168
stderr>>> 16   libswiftStdlibUnittest.dylib       0x000000010b991e40 specialized TestSuite._runTest(name:parameter:) + 550
stderr>>> 17   libswiftStdlibUnittest.dylib       0x000000010b918150 _childProcess() + 1614
stderr>>> 18   libswiftStdlibUnittest.dylib       0x000000010b91f070 runAllTests() + 688
stderr>>> 19   a.out                              0x000000010a985560 main + 36603
stderr>>> 20   libdyld.dylib                      0x00007fff8f2c4254 start + 1
stderr>>> CRASHED: SIGILL

[phausler]

@swift-ci please test

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - 2bc7b6f
Test requested by - @phausler

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - 2bc7b6f
Test requested by - @phausler

[thomasvl]

Shouldn't this have been:

return count &+ (first << lengthBits) &+ (last << (lengthBits + firstIndexBits))

i.e. if the first/last value are large, you can overflow.