[flang] Add COMDAT to global variables where needed

On platforms which support COMDAT sections we should use them when
linkonce or linkonce_odr linkage is requested. This is required on
Windows (PE/COFF) and provides better behaviour than weak symbols on
ELF-based platforms.

This patch also reverts string literals to use linkonce instead of
internal linkage now that comdats are supported.

Differential Revision: https://reviews.llvm.org/D153768