lld/coff: Implement some support for the comdat selection field

lld/coff: Implement some support for the comdat selection field

LLD used to handle comdats as if the selection field was always set to
IMAGE_COMDAT_SELECT_ANY. This means for obj files produced by `cl /Gy`, LLD
would never report a duplicate symbol error.

This change:
- adds validation for the Selection field (should make no difference in
  practice for compiler-generated obj inputs)
- rejects comdats that have different Selection fields in different obj files
  (likewise). This is a bit more strict but also more self-consistent thank
  link.exe (see comment in code)
- implements handling for all the selection kinds

In practice, compilers only generate comdats with
IMAGE_COMDAT_SELECT_NODUPLICATES (LLD now produces duplicate symbol errors for
these), IMAGE_COMDAT_SELECT_ANY (no behavior change), and
IMAGE_COMDAT_SELECT_LARGEST (for RTTI data; here LLD should no longer create
broken executables when linking some TUs with RTTI enabled and some with it
disabled – but see below).

The implementation of `IMAGE_COMDAT_SELECT_LARGEST` is incomplete: If one
SELECT_LARGEST comdat replaces an earlier one, the comdat symbol is replaced
correctly, but the old section stays loaded and if /opt:ref is disabled (via
/opt:noref or /debug) it's still written to the output. That's not ideal, but
better than the current treatment of just picking any one of those comdats. I
hope to fix this better later.

Fixes most of PR40094.

Differential Revision: https://reviews.llvm.org/D57324

llvm-svn: 352590