This fixes two major problems:
- We were not capping vector alignment as desired on 32-bit ARM.
- We were using different alignments based on the AVX settings on Intel, so we did not have a consistent ABI.
This is an ABI break, but we think we can get away with it because vectors tend to be used mostly in inline code (which is why not having a consistent ABI has not proven disastrous on Intel).
Intel's AVX types are specified as having 32-byte / 64-byte alignment, so align them explicitly instead of relying on the base ABI rule. Note that this sort of attribute is stripped from template arguments in template substitution, so there's a possibility that code templated over vectors will produce inadequately-aligned objects.
Some of our discussion leading into this change is here: https://github.com/apple/swift/pull/15691