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

[Aaron Ballman via cfe-commits]

AaronBallman 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) Or am I misreading that thread?

I get the same reading, but I am not certain we want to follow suit unless there's some significant fallout from this change (which I don't expect). Disabling dollars in identifiers in all language modes is a more consistent behavior for the extension instead of having it depend on the language mode.

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