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

[John McCall via cfe-commits]

rjmccall wrote:

Great, thanks!  @Midar, if you can fix this code up so that these strings are created in one place (and test the new output, since this is not NFC), I'd be happy to sign off.

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