Use more LLVM_ENABLE_ABI_BREAKING_CHECKS in Error.h

Summary:
If we don't read the flag, we don't need to set it. Then we also don't
need to filter it out to get the pointer.

This should have a (small) positive impact on code size and performance.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D76259