Module selectors (formerly module qualifiers)

[beccadax]

This pull request begins to implement "module selectors", which allow references to declarations to be unambiguously prefixed with a module they can be found through:

class MyClass: ObjectiveC::NSObject {}  // OK, ObjectiveC defines NSObject
class MyClass: Foundation::NSObject {}  // OK, Foundation re-exports NSObject
class MyClass: Swift::NSObject {}       // error, Swift does not re-export or define NSObject
class MyClass: MyModule::NSObject {}    // error, MyModule does not re-export or define NSObject

This PR parses module selectors and partially implements lookup and diagnostics for them. The new functionality is hidden behind an -enable-experimental-module-selector frontend flag. There is one diagnostic regression with that flag enabled, but I've made sure that it keeps working when the diagnostic is disabled.

I will need to switch to other tasks soon, so I plan to merge this work in its current incomplete state to keep it from rotting in a branch.

Makes progress on rdar://problem/19481048.

[beccadax]

I’m unfortunately running out of time to work on this, so I’ll need clean it up and land it behind a feature flag to keep it from rotting.

[beccadax]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - beda9d1ef919d7c43fc2030b404641b3d80bc461

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - beda9d1ef919d7c43fc2030b404641b3d80bc461

[beccadax]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 04f95bf3da208e0627cb695eaa71b88fb072b764

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 04f95bf3da208e0627cb695eaa71b88fb072b764

[beccadax]

#28936 broke this branch; force-pushed to correct the code.

[beccadax]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 04f95bf3da208e0627cb695eaa71b88fb072b764

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 04f95bf3da208e0627cb695eaa71b88fb072b764

[shahmishal]

Please update the base branch to main by Oct 5th otherwise the pull request will be closed automatically.

• How to change the base branch: (Link)
• More detail about the branch update: (Link)