[SveEmitter] Add SVE tuple types and builtins for svundef.

This patch adds new SVE types to Clang that describe tuples of SVE
vectors. For example `svint32x2_t` which maps to the twice-as-wide
vector `<vscale x 8 x i32>`. Similarly, `svint32x3_t` will map to
`<vscale x 12 x i32>`.

It also adds builtins to return an `undef` vector for a given
SVE type.

Reviewers: c-rhodes, david-arm, ctetreau, efriedma, rengolin

Reviewed By: c-rhodes

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81459