Use C++11 fallthrough attribute syntax when available and add a break

Summary:
This silences the two -Wimplicit-fallthrough warnings clang finds in
ItaniumDemangle.h in libc++abi.

Clang does not have a GNU attribute spelling for this attribute, so this
is necessary.

I will commit the same change to the LLVM demangler soon.

Reviewers: EricWF, ldionne

Subscribers: christof, erik.pilkington, cfe-commits

Differential Revision: https://reviews.llvm.org/D53985

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@345870 91177308-0d34-0410-b5e6-96231b3b80d8