Fix escaping in RewriterGen.cpp.

[matthiaskramm]

Summary

When we escape strings for C++, make sure we use C++ escape
sequences. (In particular, \x22 instead of \22)

[matthiaskramm]

Jeff, could you land this one for me, as well?

mlir/tools/mlir-tblgen/RewriterGen.cpp

 struct format_provider<mlir::tblgen::Pattern::IdentifierLine> {
   static void format(const mlir::tblgen::Pattern::IdentifierLine &v,
                      raw_ostream &os, StringRef style) {
     os << v.first << ":" << v.second;
   }
 };
 } // end namespace llvm
 
+// Escape a string for use inside a C++ literal.
+// E.g. foo"bar -> foo\x22bar.
 static std::string escapeString(StringRef value) {
   std::string ret;
   llvm::raw_string_ostream os(ret);
-  llvm::printEscapedString(value, os);
+  os.write_escaped(value, /*use_hex_escapes=*/true);
   return os.str();
 }
 
 //===----------------------------------------------------------------------===//
 // PatternEmitter
 //===----------------------------------------------------------------------===//
 
 namespace {