[Clang] Add support for scalable vectors in __builtin_reduce_* functions (#87750)

Currently, a lot of `__builtin_reduce_*` function do not support
scalable vectors, i.e., ARM SVE and RISCV V. This PR adds support for
them. The main code change is to use a different path to extract the
type from the vectors, the rest is the same and LLVM supports the reduce
functions for `vscale` vectors.

This PR adds scalable vector support for:
- `__builtin_reduce_add`
- `__builtin_reduce_mul`
- `__builtin_reduce_xor`
- `__builtin_reduce_or`
- `__builtin_reduce_and`
- `__builtin_reduce_min`
- `__builtin_reduce_max`

Note: For all except `min/max`, the element type must still be an
integer value. Adding floating point support for `add` and `mul` is
still an open TODO.