Sema: Move diagnoseMissingImportForMember() to Sema.

This will make it possible to use type checking APIs to determine the
appropriate access level for suggested imports.

NFC.

include/swift/AST/Module.h

@@ -1319,11 +1319,6 @@ inline SourceLoc extractNearestSourceLoc(const ModuleDecl *mod) {
   return extractNearestSourceLoc(static_cast<const Decl *>(mod));
 }
 
-/// If the import that would make the given declaration visibile is absent,
-/// emit a diagnostic and a fix-it suggesting adding the missing import.
-bool diagnoseMissingImportForMember(const ValueDecl *decl,
-                                    const DeclContext *dc, SourceLoc loc);
-
 } // end namespace swift
 
 #endif