Implement experimental module selectors (MyMod::someName) feature

[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.

This work was previously in PR #28834, which was closed during the master-to-main transition. I've updated it to work on modern compilers, including removing some FIXMEs for bad diagnostics that have improved since December 2019.

Makes progress on rdar://problem/19481048.

[nkcsgexi]

Woohoo! Thank you for reviving this, Becca!