[Modules] A utility to identify compilations that produce BMIs.

[iains]

Summary

We want to be able to decide if a given compilation should emit a
BMI if an appropriate module line is found. We must not confuse
these cases with an implementation that implicitly pulls in its
respective interface.

[iains]

rebased, updated the description to use BMI instead of CMI.

[iains]

this will be updated and moved to a pull request

clang/include/clang/Sema/Sema.h

public:
   }
 
   /// Make a merged definition of an existing hidden definition \p ND
   /// visible at the specified location.
   void makeMergedDefinitionVisible(NamedDecl *ND);
 
   bool isModuleVisible(const Module *M, bool ModulePrivate = false);
 
+  /// Determine if the current module scope is the implementation.
+  bool isModuleImplementation() const {
+    return ModuleScopes.empty() ? false : !ModuleScopes.back().ModuleInterface;
+  }
+
   // When loading a non-modular PCH files, this is used to restore module
   // visibility.
   void makeModuleVisible(Module *Mod, SourceLocation ImportLoc) {
     VisibleModules.setVisible(Mod, ImportLoc);
   }
 
   /// Determine whether a declaration is visible to name lookup.
   bool isVisible(const NamedDecl *D) {