[DebugInfo] Fix register variables not showing up in pdb.

Previously, when linking against libcmt from the MSVC runtime,
lld-link /verbose would show "Ignoring unknown symbol record
with kind 0x1006".  It turns out this was because
TypeIndexDiscovery did not handle S_REGISTER records, so these
records were not getting properly remapped.

Patch by: Alexnadre Ganea
Differential Revision: https://reviews.llvm.org/D40919

llvm-svn: 320108