regex: Fix negative sets capturing newlines

[AntonLydike]

LLVMs llvm::Regex has the following flag enabled on all FileCheck regex matches:

enum llvm::Regex::RegexFlags::Newline = 2U
Compile for newline-sensitive matching. With this flag '[^' bracket
expressions and '.' never match newline. A ^ anchor matches the
null string after any newline in the string in addition to its normal
function, and the $ anchor matches the null string before any
newline in the string in addition to its normal function.

This means that [^ ,], combined with this flag, is actually equivalent to [^\n ,] in python. Since python doesn't have a flag like this, we approximate this through a best-effort patch that tries to add \n to negative sets in the regexes as part of the regex translation.

This is not a good fix imho, as it will break on regexes where [^ appears inside a set, but that should be sufficiently rare that it won't happen 🤞