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

[Richard Smith via cfe-commits]

================
@@ -1,5 +1,5 @@
 // REQUIRES: x86-registered-target
-// RUN: %clang_cc1 %s -triple i386-apple-darwin10 -fasm-blocks -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 %s -triple i386-apple-darwin10 -fdollars-in-identifiers -fasm-blocks -emit-llvm -o - | FileCheck %s
----------------
zygoloid wrote:

Just musing... if we wanted this to work automatically, we could implement detection of `__asm` in the preprocessor, and enable dollars-in-identifiers when lexing inside an `__asm` block. Detection of `__asm` at that phase is also necessary to properly handle `;` comments, which right now [we incorrectly discard after preprocessing](https://godbolt.org/z/GjxGhY889). (I'm not sure if there are any other lexing differences for MS `__asm` that we don't properly handle, but there might be.) But it's probably not worth it for a feature that only works (in MSVC at least) when targeting 32-bit x86. :)

Perhaps a better idea would be to enable `-fdollars-in-identifiers` under `-fms-compatibility`, given that MSVC still enables support for dollars in identifiers even in its conforming mode. Though perhaps we can leave that option as a fallback for if someone actually complains.

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