[clang] cast operator from ext_vector_type of bool to an integer bitmask

Clang lacks an operator for converting SIMD vectors (ext_vector_type) consisting of bool elements to an integer type (and vice versa), for example to unsigned _BitInt(N). In LLVM IR this is done trivially, but in C such an operator does not exist. I propose adding it; currently, one has to use the __builtin_memcpy_inline builtin to do this.

Start at Clang's ext_vector_type cast handling and the __builtin_memcpy_inline workaround; compare the requested C conversion with the corresponding LLVM IR behavior. Done means conversions between bool ext_vector_type vectors and integer bitmasks, including unsigned _BitInt(N), are specified and supported in both directions.

Written by the indexing model from the issue text.