Check -Werror options during module validation

This patch checks whether the diagnostic options that could lead to
errors (principally -Werror) are consistent between when a module was
built and when it is loaded.  If there are new -Werror flags, then the
module is rebuilt.  In order to canonicalize the options we do this
check at the level of the constructed DiagnosticsEngine, which contains
the final set of diag to diagnostic level mappings.  Currently we only
rebuild with the new diagnostic options, but we intend to refine this in
the future to include the union of the new and old flags, since we know
the old ones did not cause errors.  System modules are only rebuilt when
-Wsystem-headers is enabled.

One oddity is that unlike checking language options, we don’t perform
this diagnostic option checking when loading from a precompiled header.
The reason for this is that the compiler cannot rebuild the PCH, so
anything that requires it to be rebuilt effectively leaks into the build
system.  And in this case, that would mean the build system
understanding the complex relationship between diagnostic options and
the underlying diagnostic mappings, which is unreasonable.  Skipping the
check is safe, because these options do not affect the generated AST.
You simply won’t get new build errors due to changed -Werror options
automatically, which is also true for non-module cases.