This makes it easier to use GTest in projects that build with the
-Wmissing-declarations warning. This fixes the warning in headers and
source files, though not GTest's own tests as it is rather noisy there.
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.
