[COFF] Generate a codeview build id signature for MinGW even when not creating a PDB

GNU ld, which doesn't generate PDBs, can optionally generate a
build id by passing the --build-id option. LLD's MinGW frontend knows
about this option but ignores it, as I had falsely assumed that LLD
already generated build IDs even in those cases.

If debug info is requested and no PDB path is set, generate a
build id signature as a hash of the binary itself. This allows
associating a binary to a minidump, even if debug info isn't
written in PDB form by the linker.

Differential Revision: https://reviews.llvm.org/D54828

llvm-svn: 347645