lld-link: Allow mixing 'discard' and 'largest' comdat selections

cl.exe and clang-cl.exe put vftables in a 'discard' comdat when building with
RTTI disabled (/GR-) but in a 'largest' comdat when building with RTTI enabled.
To be able to link /GR- code with /GR code, lld-link needs to accept comdats
that have this type of comdat selection conflict.

For example, static libraries in the Visual Studio standard library are built
with /GR, and without this it's impossible to build client code with /GR- and
still link to the standard library.

link.exe also accepts merging 'discard' with 'largest', and it accepts merging
'largest' with any other selection type. lld-link is still a bit stricter since
it only allows merging 'largest' with 'discard' for symmetry.

Differential Revision: https://reviews.llvm.org/D57515

llvm-svn: 352765