Wrap clang module files in a Mach-O, ELF, or COFF container.

Wrap clang module files in a Mach-O, ELF, or COFF container.
This is a necessary prerequisite for debugging with modules.
The .pcm files become containers that hold the serialized AST which allows
us to store debug information in the module file that can be shared by all
object files that were built importing the module.

rdar://problem/19104245

llvm-svn: 230044