[AArch64] Reserve a 16 byte aligned amount of fixed stack for win64 varargs

[AArch64] Reserve a 16 byte aligned amount of fixed stack for win64 varargs

Create a dummy 8 byte fixed object for the unused slot below the first
stored vararg.

Alternative ideas tested but skipped: One could try to align the whole
fixed object to 16, but I haven't found how to add an offset to the stack
frame used in LowerWin64_VASTART.

If only the size of the fixed stack object size is padded but not the offset, via
MFI.CreateFixedObject(alignTo(GPRSaveSize, 16), -(int)GPRSaveSize, false),
PrologEpilogInserter crashes due to "Attempted to reset backwards range!".

This fixes misconceptions about where registers are spilled, since
AArch64FrameLowering.cpp assumes the offset from fixed objects is
aligned to 16 bytes (and the Win64 case there already manually aligns
the offset to 16 bytes).

This fixes cases where local stack allocations could overwrite callee
saved registers on the stack.

Differential Revision: https://reviews.llvm.org/D35720

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308950 91177308-0d34-0410-b5e6-96231b3b80d8