Fix bug 34608 by moving private header out of public header.

WindowsManifestMerger.h should not include llvm/Config/config.h, since it is private.  The include has been moved to the source instead.

Summary:
The checksums had already been placed in the IR, this patch allows
MCCodeView to actually write it out to an MCStreamer.

Move private config.h header dependency out of public header file.

Addresses Bug 34608

Subscribers: javed.absar, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D37863

llvm-svn: 313312