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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Could you please also add an LLVM IR test along the lines of [llvm.godbolt.org/z/Ea7WzdczE](https://llvm.godbolt.org/z/Ea7WzdczE), so we can see the escaping is now correct?

Thx. Added llvm/test/CodeGen/X86/symbol-name.ll

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