[clang] [CIR] Add support for emitting predefined expressions (PR #161757)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I realize you're picking the description of predefinedexpr out of the documentation (though pretty sure you got it wrong :D) , but these are the 'special' macro-like-things that tell you SOMETHING about the name of a function, etc.
> 
> Do you mean my PR description got it wrong? I can expand it a bit to say something meaningful I suppose. Otherwise, my description actually does reference `__func__` but the GitHub interface interpreted the underscores as bold markup.

Right, yeah, expouse a little and use the tildes to make sure it doesn't eat the underscores.  Future people will thank us :) 

https://github.com/llvm/llvm-project/pull/161757