This is the ext_vector_type alternative to D81083.
This patch extends Clang to allow 'bool' as a valid vector element type (attribute ext_vector_type) in C/C++.
This is intended as the canonical type for SIMD masks and facilitates clean vector intrinsic declarations.
Vectors of i1 are supported on IR level and below down to many SIMD ISAs, such as AVX512, ARM SVE (fixed vector length) and the VE target (NEC SX-Aurora TSUBASA).
The RFC on cfe-dev: https://lists.llvm.org/pipermail/cfe-dev/2020-May/065434.html