Avoid evaluating Neon macro arguments more than once by disabling type checks.

It turns out that the use of "__extension__" in these macros was disabling
the expected "incompatible pointer" warnings, so these type checks were not
doing anything anyway.  They introduced a serious bug by evaluating some
macro arguments twice, which is a big problem for arguments with side effects.
I'll have to find another way to get the right type checking.  Radar 9947657.

llvm-svn: 137680