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.

lib/AST/Module.cpp

@@ -2051,6 +2051,8 @@ bool SourceFile::isImportedImplementationOnly(const ModuleDecl *module) const {
 }
 
 bool ModuleDecl::isImportedImplementationOnly(const ModuleDecl *module) const {
+  if (module == this) return false;
+
   auto &imports = getASTContext().getImportCache();
 
   // Look through non-implementation-only imports to see if module is imported