[lld][WebAssembly] Do not merge comdat data segments

When running in relocatable mode any input data segments that are part
of a comdat group should not be merged with other segments of the same
name.  This is because the final linker needs to keep the separate so
they can be included/excluded individually.

Often this is not a problem since normally only one section with a given
name `foo` ends up in the output object file.  However, the problem
occurs when one input contains `foo` which part of a comdat and another
object contains a local symbol `foo` we were attempting to merge them.

This behaviour matches (I believe) that of the ELF linker.  See
`LinkerScript.cpp:addInputSec`.

Fixes: https://github.com/emscripten-core/emscripten/issues/9726

Differential Revision: https://reviews.llvm.org/D101703