According to the IR verifier, "Declaration[s] may not be in a Comdat!"
This is a re-commit of 76b3f0b4d5 and
87d7838202 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
50ca8b3e87