[AArch64] Fix calling windows varargs with floats in fixed args from non-windows...

[AArch64] Fix calling windows varargs with floats in fixed args from non-windows functions

When inspecting the calling convention, for calling windows functions
from a non-windows function, inspect the calling convention of
the called function, not the caller.

Also remove an unnecessary parameter to AArch64CallLowering
OutgoingArgHandler.

Differential Revision: https://reviews.llvm.org/D100890