[AArch64][SVE] SVE functions should use the SVE calling convention for fast calls

When an SVE function calls another SVE function using the C calling
convention we use the more efficient SVE VectorCall PCS.  However,
for the Fast calling convention we're incorrectly falling back to
the generic AArch64 PCS.

This patch adds the same "can use SVE vector calling convention"
detection used by CallingConv::C to CallingConv::Fast.

Co-authored-by: Paul Walker <paul.walker@arm.com>

Differential Revision: https://reviews.llvm.org/D99657