[Clang][AArch64] Inline assembly support for the ACLE type 'data512_t'

In LLVM IR terms the ACLE type 'data512_t' is essentially an aggregate
type { [8 x i64] }. When emitting code for inline assembly operands,
clang tries to scalarize aggregate types to an integer of the equivalent
length, otherwise it passes them by-reference. This patch adds a target
hook to tell whether a given inline assembly operand is scalarizable
so that clang can emit code to pass/return it by-value.

Differential Revision: https://reviews.llvm.org/D94098