[modules] Allow -I, -D, -W flags to change between building a module and
explicitly using the resulting .pcm file. Unlike for an implicit module build,
we don't need nor want to require these flags to match between the module
and its users.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220780 91177308-0d34-0410-b5e6-96231b3b80d8