[NativePDB] Add support for S_CONSTANT records.


[NativePDB] Add support for S_CONSTANT records.

clang-cl does not emit these, but MSVC does, so we need to be able to
handle them.

Because clang-cl does not generate them, it was a bit hard to write a
test. So what I had to do was get an PDB file with some S_CONSTANT
records in using cl and link, dump it using llvm-pdbutil dump -globals
-sym-data to get the bytes of the records, generate the same object file
using clang-cl but with -S to emit an assembly file, and replace all the
S_LDATA32 records with the bytes of the S_CONSTANT records. This way, we
can compile the file using llvm-mc and link it with lld-link.

Differential Revision: https://reviews.llvm.org/D54452