Due to an unstable (and undesirable) calling convention in the LLVM layer for x86, I had previously marked Float16 unconditionally unavailable on macOS. My hope was that Intel would stabilize the calling convention and we could make it available on both macOS platforms at the same time. Unfortunately, that hasn't happened, and we want to make the type available for macOS/arm users.
So, I am making the availability mirror Float80--the type will be unavailable for macOS on x86_64, and available on all other platforms (the other x86 platforms don't have a binary-stability guarantee to worry about). This isn't ideal. In particular, if/when the calling conventions for Float16 stabilize in LLVM, we would want to make the type available, but it would then have _different_ availability for different architectures of macOS, which the current availability system is not well-equipped to handle (it's possible, but not very ergonomic). Nonetheless, this seems like the best option.
The good news is that because the full API is already built in Swift (and simply marked unavailable), we can simply add macOS 11.0 availability for these API and it will work.
* Teach the importer to import any vector type as SIMDN<Scalar>.
Instead of having a known set of vector types, check to see if the
element type conforms to SIMDScalar; if it does, see if we have a
SIMDN defined with the right number of elements. If both are satisfied,
import the vector type as that Swift type.
By making this change, we gain the ability to import vector types
that aren't defined in terms of the Darwin simd module, which lets
us use C API with vector types on other platforms. It also lets us
import *every* vector type that Swift can represent, rather than the
small subset that are currently hardcoded.
* Increased test coverage for increased SIMD types that we can import.
Includes some minor cleanup from review. Also eliminates the old
simd_sans_simd test, since we can now import all of these types even when the simd module isn't imported.
