[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

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320719 91177308-0d34-0410-b5e6-96231b3b80d8