[llvm] [llvm-mc][AsmParser] Fix Source location for undefined directional labels. (PR #214654)

[via llvm-commits]

zhangweize9-cyber wrote:

Hi! @lenary 

My initial approach was to add a `DenseMap` and use `LocalSymbolLocs.find()` to filter the results. 

Let me know if this looks good to you or if you have a better idea in mind!

https://github.com/llvm/llvm-project/pull/214654