[libcxxabi] Don't process exceptions in cxa_handlers when they're disabled

When exceptions are disabled, avoid their processing altogether.
This avoids pulling in the depenency on demangler significantly
reducing binary size when statically linking against libc++abi
built without exception support.

Differential Revision: https://reviews.llvm.org/D64191

llvm-svn: 365944
GitOrigin-RevId: 2e398f1895b9ec154718fa03655f625e7848fff6
3 files changed