The recently added pragma to suppress warnings about unused local
typedefs caused lots of dashboard failures because many GCC and clang
compiler do not have this warning so did not recognized the pragma to
suppress it. Now only use the pragma on clang compilers with a large
enough version.

I also discovered that the check for VTKM_CLANG was wrong (at least for
the most modern versions of XCode). Fixed that as well as some uses of
VTKM_CLANG that were wrong.