Update Swift-mangled NSCFConstantString class name

[e78l]

Fixing:

ld: warning: undefined base symbol '__TMC15SwiftFoundation19_NSCFConstantString' for alias '___CFConstantStringClassReference'
Undefined symbols for architecture x86_64:
  "__T015SwiftFoundation21__CFSwiftGetBaseClassyXlXpyF", referenced from:
      ___CFInitialize in libCoreFoundation.a(CFRuntime.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Split from #988 (this has somewhat higher risk of breaking on some Macs, maybe)

[parkera]

Just to be clear on this: this is the mangled name produced by the top of tree Swift compiler right?

[e78l]

Hello Tony,
Thanks for asking - I use snapshots on macOS, and when checking with swift-DEVELOPMENT-SNAPSHOT-2017-05-23-a, only the string warning appears:

ld: warning: undefined base symbol '__TMC15SwiftFoundation19_NSCFConstantString' for alias '___CFConstantStringClassReference'

Looks like the AnyObject part of the class name isn't in the Swift 4 branch. (When I submitted the PR, I was using the 5/15 swift 4 snapshot)

So, I've dropped the CFRuntime init name change, but kept the SymbolAliases change.
As I mentioned in #988, I think it would be better to have a script determine these mangled names at build time, but I'm not sure how to get them.

[parkera]

I think we're actually going to be ok for the long term once Swift 4 ships - changing mangling is a big deal and I'm hoping it calms down now.