[MSVC, ARM64] Add __writex18 intrinsics

https://docs.microsoft.com/en-us/cpp/intrinsics/arm64-intrinsics?view=msvc-170

  void __writex18byte(unsigned long, unsigned char)
  void __writex18word(unsigned long, unsigned short)
  void __writex18dword(unsigned long, unsigned long)
  void __writex18qword(unsigned long, unsigned __int64)

Given the lack of documentation of the intrinsics, we chose to align the offset with just
`CharUnits::One()` when calling `IRBuilderBase::CreateAlignedStore()`.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D126023