Plumbing for a Virtual File System

Adds the -vfsoverlay frontend option that enables the user to pass
VFS overlay YAML files to Swift. These files define a (potentially
many-layered) virtual mapping on which we predicate a VFS.

Switch all input-based memory buffer reads in the Frontend to the new
FileSystem-based approach.

lib/Frontend/CompilerInvocation.cpp

@@ -463,6 +463,10 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts,
     Opts.LibrarySearchPaths.push_back(resolveSearchPath(A->getValue()));
   }
 
+  for (const Arg *A : Args.filtered(OPT_vfsoverlay)) {
+    Opts.VFSOverlayFiles.push_back(resolveSearchPath(A->getValue()));
+  }
+
   if (const Arg *A = Args.getLastArg(OPT_sdk))
     Opts.SDKPath = A->getValue();