[AArch64] Treat x18 as callee-saved in functions with Windows calling convention on Darwin

rGcf97e0ec42b8 makes $x18 to be treated as callee-saved in functions with
Windows calling convention on non-Windows OSes.

Here we mark $x18 as callee-saved for functions with Windows calling
convention on Darwin, as well as on other non-Windows platforms, in
order to prevent some miscompilations (like miscompilation of
win64cc-darwin-backup-x18.ll).

Since getCalleeSavedRegs doesn't return x18 in list of callee-saved
registers, assignCalleeSavedSpillSlots and determineCalleeSaves
consider different sets of registers as callee-saved. It causes an
error:
```
Assertion failed: ((!HasCalleeSavedStackSize || getCalleeSavedStackSize() == Size) && "Invalid size calculated for callee saves"), function getCalleeSavedStackSize, file
AArch64MachineFunctionInfo.h, line 292.
```

Differential Revision: https://reviews.llvm.org/D130676