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

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R-Goc wrote:

It appears that gcc only disallows '&' in identifiers in C++26 mode. Not sure how relevant that is to clang. [bugzilla](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110343)

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