Force #define GTEST_LANG_CXX11.

Force #define GTEST_LANG_CXX11.

gtest depends on this #define to determine whether it can
use various classes like std::tuple, or whether it has to fall
back to experimental classes in the std::tr1 namespace.  The
check in the current version of gtest relies on the value of
the `__cplusplus` macro, but MSVC provides a non-conformant
value of this macro, making it effectively impossible to detect
C++11.  In short, LLVM compiled with MSVC has been silently
using the tr1 versions of several classes since the beginning of
time.

This would normally be pretty benign, except that in the latest
preview of MSVC they have marked all of the tr1 classes
deprecated, so it spews thousands of warnings.

llvm-svn: 316798