[llvm] MC: Better handle backslash-escaped symbols (PR #158780)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: 76aba5d415fbf206e0d9443a5822fcd9244fa33f

https://github.com/llvm/llvm-project/actions/runs/17802897787

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/158780