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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> Effectively checking for `LangOpts.DollarIdents || LangOpts.AsmPreprocessor`? Or do you think `DollarIdents` should be true if `AsmPreprocessor` is true?

Neither.  DollarIdents is already off by default in AsmPreprocessor mode.  We should just preserve the existing behavior, and not warn.

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(I'm not I really understand all the standards-committee stuff, but it seems dubious to have the semantics of $ in preprocessor macros implementation-defined.)

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