[SVE] Ensure proper mangling of ACLE tuple types

The AAPCS specifies that the tuple types such as `svint32x2_t`
should use their `arm_sve.h` names when mangled instead of their
builtin names.

This patch also renames the internal types for the tuples to
be prefixed with `__clang_`, so they are not misinterpreted as
specified internal types like the non-tuple types which *are* defined
in the AAPCS. Using a builtin type for the tuples is a purely
a choice of the Clang implementation.

Reviewers: rsandifo-arm, c-rhodes, efriedma, rengolin

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81721