Fixed -Wexceptions derived-to-base false positives

...as introduced with recent <https://reviews.llvm.org/D33333> "Emit warning
when throw exception in destruct or dealloc functions which has a (possible
implicit) noexcept specifier".  (The equivalent of the goodReference case hit
when building LibreOffice.)

(These warnings are apparently only emitted when no errors have yet been
encountered, so it didn't work to add the test code to the end of the existing
clang/test/SemaCXX/exceptions.cpp.)

llvm-svn: 306715