Expose FileCheck's AddFixedStringToRegEx as Regex::escape

Both FileCheck and clang's -verify need to escape strings for regexes,
so let's expose this as a utility in the Regex class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197096 91177308-0d34-0410-b5e6-96231b3b80d8