[MSVC, ARM64] Add __readx18 intrinsics

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

  unsigned char __readx18byte(unsigned long)
  unsigned short __readx18word(unsigned long)
  unsigned long __readx18dword(unsigned long)
  unsigned __int64 __readx18qword(unsigned long)

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

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D126024