[clang] Disable -fdollars-in-identifiers by default (PR #135407)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > If we're going to do this, I think we need better diagnostics. Just straight disabling this is going to give very confusing diagnostics to anyone actually using dollar-signs in identifiers.
> > Some ideas:
> > ```
> > * We can give a warning if we see a "$" adjacent to an identifier without any whitespace separating it.
> > 
> > * Outside the preprocessor, we can parse a "$" adjacent to an identifier as part of the identifier, with some sort of diagnostic, since it's guaranteed to be an error anyway.
> > ```
> 
> My concern here is with regressing performance of the lexer; testing those conditions when lexing _any_ identifier seems like we'd be spending a lot of time trying to catch a very uncommon issue.
> 
> I think this is reasonable as follow-up work, but I don't think it should hold up this PR because this is fixing a conformance issue. (FWIW, I think the behavior you get currently is something we could live with even if that follow-up work never happened.)

Are you okay with me punting on this work and landing the changes as-os @efriedma-quic ?

https://github.com/llvm/llvm-project/pull/135407