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

include/llvm/Support/Regex.h

@@ -81,6 +81,9 @@ namespace llvm {
     /// expression that matches Str and only Str.
     static bool isLiteralERE(StringRef Str);
 
+    /// \brief Turn String into a regex by escaping its special characters.
+    static std::string escape(StringRef String);
+
   private:
     struct llvm_regex *preg;
     int error;