[clang] Fix objc_sel_{name,types} missing an underscore (PR #88713)

[David Chisnall via cfe-commits]

davidchisnall wrote:

Yup, it’s just an optimisation. The runtime does a slightly more complex deduplication pass during loading, this just does a best effort merge so that we don’t end up with hundreds of copies of common selectors as we did with the GCC ABI. If we don’t do it, it isn’t an ABI break, it’s only an ABI break if we merge two strings that are not the same.

Some of this code accreted and so deduplicating the code, as well as the selectors, is a good idea.

https://github.com/llvm/llvm-project/pull/88713