[Serialization] Add a "nested types" lookup table for partial modules

[jrose-apple]

• Explanation: There's a class of errors in Serialization called "circularity issues", where declaration A in file A.swift depends on declaration B in file B.swift, and B also depends on A. A common case of this is when at least one of the declarations is nested, in which case a lookup to find that declaration needs to load all the members of the parent type. This commit sidesteps that issue specifically for nested types by creating a top-level, per-file table of nested types in the "partial modules". When a type is in the same module, we can then look it up without importing all other members of the parent type. This fixes a regression from Swift 3.0.
• Scope: Affects non-WMO builds that include references to nested types across files in the same module.
• Issue: SR-3707 / rdar://problem/30172856
• Reviewed by: @DougGregor
• Risk: Medium-low. This is a bunch of new code that changes something that happens in many builds, but there's at least a flag to turn it off.
• Testing: Added compiler regression tests, verified that the original project now passes.

[jrose-apple]

@swift-ci Please test

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - adbfbe9
Test requested by - @jrose-apple

[jrose-apple]

Failure during checkout.

@swift-ci Please test Linux