[COFF] Teach LLD to use the COFF .debug$H section.

This adds the /DEBUG:GHASH option to LLD which will look for
the existence of .debug$H sections in linker inputs and use them
to accelerate type merging.  The clang-cl side has already been
added, so this completes the work necessary to begin experimenting
with this feature.

Differential Revision: https://reviews.llvm.org/D40980

llvm-svn: 320719