[COFF] Assign unique identifiers to ObjFiles from LTO

Use the unique filenames that are used when /lldsavetemps is passed.
After this change, module names for LTO blobs in PDBs will be unique.
Visual Studio and probably other debuggers expect module names to be
unique.

Revert some changes from 1e0b158d (2017) that are no longer necessary
after removing MSVC LTO support.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D78221