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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

If you don't want to do anything fancy, can you just add a warning in Lexer::LexTokenInternal if we see a '$' (where we set the token type to tok::unknown)?  That should be easy to implement, and have zero impact on lexer performance.

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