[clang] Tracks source location of the selector name within ObjCSelectorExpr (PR #211623)

[Justin Stitt via cfe-commits]

JustinStitt wrote:

> Apologies for the commit name on the cleanup. I assume it won't matter as you are going to squash the commits.

right. commit name doesn't matter.

> Updated naming and added some tests. 

thanks for making these changes. if you don't have merge access, I'll see to merging this later today.

https://github.com/llvm/llvm-project/pull/211623