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

[Richard Smith via cfe-commits]

zygoloid wrote:

(I guess we could also warn if an object-like macro is expanded and the macro name is preceded or followed by a `$` with no whitespace, since that would also behave differently under `-fdollars-in-identifiers`, but I suspect that's not worth worrying about in practice.)

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