[AArch64][InlineAsm]Add Clang support for flag output constraints

- Mention this change in Clang release notes

Before:
- Clang emits "invalid output constraint '=@cceq' in asm" https://gcc.godbolt.org/z/b9crfEo8h

After:
- For aarch64 targets (with __aarch64__ defined), Clang validates and parses flag output constraints to generate LLVM IR.

Differential Revision: https://reviews.llvm.org/D149123