[mlir] Add support for LLVMIR comdat operation

The LLVM comdat operation specifies how to deduplicate globals with the
same key in two different object files. This is necessary on Windows
where e.g. two object files with linkonce globals will not link unless
a comdat for those globals is specified. It is also supported in the ELF
format.

Differential Revision: https://reviews.llvm.org/D150796