[ARM] Fix arm_neon.h with -flax-vector-conversions=none, part 2.
Just running -fsyntax-only over arm_neon.h doesn't cover some intrinsics
which are defined using macros. Add more test coverage for that.
arm-neon-header.c wasn't checking the full set of available NEON target
features; change the target architecture of the test to account for
Fix the generator for arm_neon.h to generate casts in more cases where
they are necessary.
Fix VFMLAL_LOW etc. to express their signatures differently, so the
builtins have the expected type. Maybe the TableGen backend should
detect intrinsics that are defined the wrong way, and produce an error.
The rules here are sort of strange.
Differential Revision: https://reviews.llvm.org/D68743