[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

llvm-svn: 369153

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