I don't know when, but at some point we lost test coverage to ensue that
all the headers are in the modulemap. This adds a test to make sure all
the headers (excluding a few which shouldn't be part of the modulemap)
are at least mentioned. This also fixes a few headers which bit-rotted
while we were missing the coverage.
[libcxx] <experimental/simd> Added internal storage type for class simd/simd_mask
[libcxx] <experimental/simd> Added all constructors of class simd/simd_mask and related tests
[libcxx] <experimental/simd> Added basic simd reference implementation, subscript operators of class simd/simd_mask and related tests
Reviewed By: #libc, philnik
Differential Revision: https://reviews.llvm.org/D144364
