[AArch64][SVE] Don't support fixedStack for SVE objects.

Fixed stack objects are preallocated and defined to be allocated before
any of the regular stack objects. These are normally used to model stack
arguments.

The AAPCS does not support passing SVE registers on the stack by value
(only by reference). The current layout also doesn't place them before
all stack objects, but rather before all SVE objects. Removing this
simplifies the code that emits the allocation/deallocation
around callee-saved registers (D84042).

This patch also removes all uses of fixedStack from from
framelayout-sve.mir, where this was used purely for testing purposes.

Reviewers: paulwalker-arm, efriedma, rengolin

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D84538