Add stopgap option -fmodule-implementation-of <name>

This flag specifies that we are building an implementation file of the
module <name>, preventing importing <name> as a module. This does not
consider this to be the 'current module' for the purposes of doing
modular checks like decluse or non-modular-include warnings, unlike
-fmodule-name.

This is needed as a stopgap until:
1) we can resolve relative includes to a VFS-mapped module (or can
   safely import a header textually and as part of a module)

and ideally
2) we can safely do incremental rebuilding when implementation files
   import submodules.

llvm-svn: 213767

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1495,6 +1495,14 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
   Opts.DebuggerObjCLiteral = Args.hasArg(OPT_fdebugger_objc_literal);
   Opts.ApplePragmaPack = Args.hasArg(OPT_fapple_pragma_pack);
   Opts.CurrentModule = Args.getLastArgValue(OPT_fmodule_name);
+  Opts.ImplementationOfModule =
+      Args.getLastArgValue(OPT_fmodule_implementation_of);
+
+  if (!Opts.CurrentModule.empty() && !Opts.ImplementationOfModule.empty() &&
+      Opts.CurrentModule != Opts.ImplementationOfModule) {
+    Diags.Report(diag::err_conflicting_module_names)
+        << Opts.CurrentModule << Opts.ImplementationOfModule;
+  }
 
   if (Arg *A = Args.getLastArg(OPT_faddress_space_map_mangling_EQ)) {
     switch (llvm::StringSwitch<unsigned>(A->getValue())