This patch adds ObjectFilePCHContainerOperations uses the LLVM backend
to put the contents of a PCH into a __clangast section inside a COFF, ELF,
or Mach-O object file container.

This is done to facilitate module debugging by makeing it possible to
store the debug info for the types defined by a module alongside the AST.

rdar://problem/20091852

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241620 91177308-0d34-0410-b5e6-96231b3b80d8