ObjectFile: introduce a COFF object file plugin

Windows uses COFF as an object file format and PE/COFF as an executable
file format. They are subtly different and certain elements of a COFF
file may not be present in an executable. Introduce a new plugin to add
support for the COFF object file format which is required to support
loading of modules built with -gmodules. This is motivated by Swift
which serialises debugging information into a PCM which is a COFF object
file.

Differential Revision: https://reviews.llvm.org/D149987
Reviewed By: bulbazord