Use std::string and ::string explicitly in gtest and gmock code.

This merges a Google-internal change (117235625).

Original CL description:
This CL was created manually in about an hour with sed, a Python script
to find all the places unqualified 'string' was mentioned, and some help
from Emacs to add the "std::" qualifications, plus a few manual tweaks.

googlemock/src/gmock-cardinalities.cc

@@ -92,7 +92,7 @@ class BetweenCardinalityImpl : public CardinalityInterface {
 };
 
 // Formats "n times" in a human-friendly way.
-inline internal::string FormatTimes(int n) {
+inline std::string FormatTimes(int n) {
   if (n == 1) {
     return "once";
   } else if (n == 2) {