[LLD][COFF] Generate import modules & COFF groups in PDB

Generate import modules for each imported DLL, along with its symbol stream.
Also create COFF groups in the * Linker * module, one for each PartialSection (input, unmerged sections)
Currently COFF groups are disabled for MINGW because it significantly increases PDB sizes. We could enable that later with an option.

The overall objective for this change is to support code hot patching tools. Such tools need to know the import libraries used, from the PDB alone.

Differential Revision: https://reviews.llvm.org/D54802

llvm-svn: 357308