[modules] Round-trip -Werror flag through explicit module build.

The intent for an explicit module build is that the diagnostics produced within
the module are those that were configured when the module was built, not those
that are enabled within a user of the module. This includes diagnostics that
don't actually show up until the module is used (for instance, diagnostics
produced during template instantiation and weird cases like -Wpadded).

We serialized and restored the diagnostic state for individual warning groups,
but previously did not track the state for flags like -Werror and -Weverything,
which are implemented as separate bits rather than as part of the diagnostics
mapping information.

llvm-svn: 301992