This patch includes:
- Remove existing modulemap.overlay files
  Cling now can generate virtual overlay files on-demand.
- Implement `-modulemap_overlay` flag to Cling
  This flag is used to tell Cling the location of modulemaps to be
  overlayed. (eg. stl.modulemap, libc.modulemap)
- Generate and load virtual modulemap in Cling
  It is in Interpreter constructor, happens as part of cxxmodules
  initialization step.
- Implement an interface to Clang CompilerInvocation which can take FileSystem pointer
  Previously, Clang only took a "string of filenames" which clearly doesn't
  work for our usecase. We already discussed this new interface at
  modules meeting, so this change will land upstream.
- Add a file existence check in Clang
  When compiling a pcm and when Clang saw the #include with which pcm was available
  (or being generated implicitly), Clang was putting a notation of the full path to this pcm.
  This caused an error when build directory was deleted, because the path didn't exist anymore.

This patch enables modules to be binary distributed, and to make it
work in CMSSW enviroment.