[Utility] Reimplement RegularExpression on top of llvm::Regex
Originally I wanted to remove the RegularExpression class in Utility and
replace it with llvm::Regex. However, during that transition I noticed
that there are several places where need the regular expression string.
So instead I propose to keep the RegularExpression class and make it a
thin wrapper around llvm::Regex.
This patch also removes the workaround for empty regular expressions.
The result is that we are now (more or less) POSIX conformant.
Differential revision: https://reviews.llvm.org/D66174