[msan] Handle aarch64_neon_vcvt* (#177243)
This fills in missing gaps in MSan's AArch64 NEON vector conversion
intrinsic handling (intrinsics named aarch64_neon_vcvt* instead of
aarch64_neon_fcvt*). SVE support sold separately.
It also generalizes handleNEONVectorConvertIntrinsic to handle
conversions to/from fixed-point.
