[ARM] Fix arm_neon.h with -flax-vector-conversions=none

Really, we were already 99% of the way there; just needed a couple minor
fixes that affected 64-bit-only builtins.  Based on D61717.

Note that the change to builtin_str changes the type of a few
__builtin_neon_* intrinsics that had the "wrong" type.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43341

Differential Revision: https://reviews.llvm.org/D68683

llvm-svn: 374191