Add missing escape characters to the new Regex::escape() function

The old AddFixedStringToRegEx() it was based on got away with this for the
longest time, but the problem became easy to spot after the cleanup in r197096.

Also add a quick unit test to cover regex escaping.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197121 91177308-0d34-0410-b5e6-96231b3b80d8