[ORC] Drop Comdat when discarding IR symbol

According to the IR verifier, "Declaration[s] may not be in a Comdat!"

This is a re-commit of 76b3f0b4 and
87d78382 with updates to the test:
 * Force emission of the extra-module, to trigger the bug after D138264,
   by providing a second symbol @g, and making the comdat nodeduplicate.
   (Technically only one is needed, but two should be safer.)
 * Name the comdat $f to avoid failure on Windows:
   LLVM ERROR: Associative COMDAT symbol 'c' does not exist.
 * Mark the test as UNSUPPORTED on macOS, MachO doesn't support COMDATs.

Differential Revision: https://reviews.llvm.org/D142443