make -fmodules-codegen and -fmodules-debuginfo work also with PCHs

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 cbc9d22e,
but then got reverted in 7ea9a6e0
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.

Differential Revision: https://reviews.llvm.org/D69778