Remove two fall through diagnostics from the IgnoredAttributes group, since...

Remove two fall through diagnostics from the IgnoredAttributes group, since they are errors, not warnings.

Only warnings should be in warning groups.  This was causing an assertion failure in
code using pragmas to map -Wignored-attributes to ignored, but this kind of
diagnostic mapping is disallowed for builtin errors.  We should just wire
up tablegen to reject errors being in diagnostic groups.

llvm-svn: 161376