Preserve the "last diagnostic was suppressed" flag across SFINAE checks.

Sometimes we check the validity of some construct between producing a
diagnostic and producing its notes. Ideally, we wouldn't do that, but in
practice running code that "cannot possibly produce a diagnostic" in such a
situation should be safe, and reasonable factoring of some code requires it
with our current diagnostics infrastruture. If this does happen, a diagnostic
that's suppressed due to SFINAE should not cause notes connected to the prior
diagnostic to be suppressed.

llvm-svn: 319408