Allow to build PCH's (with -building-pch-with-obj and the extra .o file) with -fmodules-codegen -fmodules-debuginfo to allow emitting shared code into the extra .o file, similarly to how it works with modules. A bit of a misnomer, but the underlying functionality is the same. This saves up to 20% of build time here. The patch is fairly simple, it basically just duplicates -fmodules checks to also alternatively check -building-pch-with-obj.
This already got committed as cbc9d22e49b434b6ceb2eb94b67079d02e0a7b74, but then got reverted in 7ea9a6e0220da36ff2fd1fbc29c2755be23e5166 because of PR44953, as discussed in D74846. This is a corrected version which does not include two places for the PCH case that aren't included in the modules -fmodules-codegen path either.