For PR11916: Add support for g++'s __int128 keyword. Unlike __int128_t, this is

a type specifier and can be combined with unsigned. This allows libstdc++4.7 to
be used with clang in c++98 mode.

Several other changes are still required for libstdc++4.7 to work with clang in
c++11 mode.

llvm-svn: 153999