Add a driver option -ivfsoverlay

Reads the description of a virtual filesystem from a file and overlays
it over the real file system.

llvm-svn: 202176

clang/lib/Frontend/CompilerInvocation.cpp

@@ -1020,6 +1020,10 @@ static void ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args) {
        I != E; ++I)
     Opts.AddSystemHeaderPrefix((*I)->getValue(),
                                (*I)->getOption().matches(OPT_isystem_prefix));
+
+  for (arg_iterator I = Args.filtered_begin(OPT_ivfsoverlay),
+       E = Args.filtered_end(); I != E; ++I)
+    Opts.AddVFSOverlayFile((*I)->getValue());
 }
 
 void CompilerInvocation::setLangDefaults(LangOptions &Opts, InputKind IK,