[CodeView] Teach clang to emit the .debug$H COFF section.

Currently this is an LLVM extension to the COFF spec which is
experimental and intended to speed up linking.  For now it is
behind a hidden cl::opt flag, but in the future we can move it
to a "real" cc1 flag and have the driver pass it through whenever
it is appropriate.

The patch to actually make use of this section in lld will come
in a followup.

Differential Revision: https://reviews.llvm.org/D40917

llvm-svn: 320649