Warn about module name shadowing in interfaces

There is a known issue with module interfaces where a type with the same name as a module will disrupt references to types in that module. Fully fixing it will require a new language feature (SR-898) which is not yet available. In the meantime, module interfaces support a workaround flag (“-Xfrontend -module-interface-preserve-types-as-written”) which prints an alternate form that usually works. However, you have to know to add this flag, and it’s not obvious because nothing breaks until a compiler tries to consume the affected module interface (or sometimes even one of its clients).

This commit emits a warning during module interface emission whenever the module interface either imports a type with the same name as the module being built, or declares a type with the same name as a visible module. This lets the user know that the type may cause problems and they might need to implement a workaround.

test/ModuleInterface/Inputs/TestModule.swift

@@ -1,3 +1,5 @@
 public struct TestStruct {
   public init() {}
 }
+
+public enum module_shadowing {}