Allow one to separately specify the ABI name of a module.

Introduce a new compiler flag `-module-abi-name <name>` that uses the
given name as the ABI name for the module (rather than the module's
name in source code). The ABI name impacts name mangling and metadata.

lib/Serialization/ModuleFileSharedCore.h

@@ -73,6 +73,9 @@ class ModuleFileSharedCore {
   /// include the version string of the compiler that built the module.
   StringRef MiscVersion;
 
+  /// The module ABI name.
+  StringRef ModuleABIName;
+
   /// \c true if this module has incremental dependency information.
   bool HasIncrementalInfo = false;