[clang] [Clang][AArch64] Define __USER_LABEL_PREFIX__ to # for ARM64EC (PR #78913)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I can sort of see how a define could be useful, but this doesn't match any existing usage of __USER_LABEL_PREFIX__ given the relevant rules. I think reusing the name is more likely to cause confusion, rather than help anyone.

https://github.com/llvm/llvm-project/pull/78913