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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Doesn't the Linux kernel use it for syscall names? Might be wrong on this.

CC @nathanchance @nickdesaulniers for more information, but even if the kernel does, is it a significant imposition to pass `-fdollars-in-identifiers` to enable the extension explicitly?

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