gtest: remove recursive make, add python driver + Xcode error hook-up.

This change does the following:
* Removes the gtest/Makefile recursive-make-based calling strategy
  for gtest execution.
* Adds the gtest/do-gtest.py call script.
  - This handles finding and calling the Makefiles that really
    run tests.
  - This script also transforms the test output into something
    that Xcode can place a failure marker on when a test fails.
* Modifies the Xcode external build command target for gtest.
  It now calls the gtest/do-gtest.py script.

There is still room for improvement on Xcode integration of
do-gtest.py.  Essentially the next several lines of error reporting
from the gtest output should be coalesced into a single line so that
Xcode can tell more about the error directly in the editor.  Right now
it just puts a red mark and says "failure" but doesn't give any
details.

llvm-svn: 218470