function multi-versioning: internal linkage resolvers should not use comdat

[MaskRay]

For function multi-versioning using the target and target_clones
function attributes, currently we incorrectly set comdat for internal
linkage functions. This is problematic for ELF linkers
as GRP_COMDAT deduplication will kick in even with STB_LOCAL signature
(https://groups.google.com/g/generic-abi/c/2X6mR-s2zoc
"GRP_COMDAT group with STB_LOCAL signature").

In short, two __attribute((target_clones(...))) static void foo()
in two translation units will be deduplicated.

[llvmbot]

@llvm/issue-subscribers-clang-codegen