[clang] [clang] Remove fixed point arithmetic error (PR #71884)

[John McCall via cfe-commits]

rjmccall wrote:

I'm happy with the basic idea here of making fixed-point an opt-in feature.  It'd be nice to preserve the special diagnostic that tells the user that they need to use `-ffixed-point`, though.  Do we have any existing parsing paths that recognize unrecognized identifiers that are written as apparent type specifiers?

https://github.com/llvm/llvm-project/pull/71884