[AArch64] Add AArch64 Neon intrinsics test produced by emperor

The test has a basic correctness check for every AArch64 Neon intrinsic,
except for a small number of intrinsics that emperor currently doesn't support
yet.

This adds a new sub-directory SingleSource/UnitTests/Vector/AArch64, as there
wasn’t a sub-directory yet for AArch64-specific test cases.



git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@221255 91177308-0d34-0410-b5e6-96231b3b80d8