[NFC] refactor demangle of llvm-nm (#67481)

since we landed the patch `https://reviews.llvm.org/D139864


[llvm-cxxfilt] Do not consider the prefix dot as part of the demangled
symbol name`. we can simply the code on the llvm-nm demanlge by using
the llvm::demangle directly.

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Co-authored-by: zhijian <zhijian@ca.ibm.com>