[clang][modules] Only compute affecting module maps with implicit search (#87849)

When writing out a PCM, we compute the set of module maps that did
affect the compilation and we strip the rest to make the output
independent of them. The most common way to read a module map that is
not affecting is with implicit module map search. The other option is to
pass a bunch of unnecessary `-fmodule-map-file=<path>` arguments on the
command-line, in which case the client should probably not give those to
Clang anyway.

This makes serialization of explicit modules faster, mostly due to
reduced file system traffic.