[MSVC, ARM64] Add __readx18 intrinsics

[steplong]

Summary

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()

[steplong]

Like D126023, the generated assembly for MSVC is slightly different:

MSVC                                                                          LLVM
__readx18byte:
                          ldrb    w0, [x18, x0]                             ldrb    w0, [x18, w0, uxtw] 
__readx18word:
                          ldrh    w0, [x18, x0]                             ldrh    w0, [x18, w0, uxtw #1]
__readx18dword:
                          ldr     w0, [x18, x0]                             ldr     w0, [x18, w0, uxtw #2]
__readx18qword:
                          ldr     x0, [x18, x0]                              ldr     x0, [x18, w0, uxtw #3]

[steplong]

The assembly for LLVM is now:

__readx18byte:
       ldrb    w0, [x18, w0, uxtw]
__readx18word:
       ldrh    w0, [x18, w0, uxtw]
__readx18dword:
       ldr     w0, [x18, w0, uxtw]
__readx18qword:
       ldr     x0, [x18, w0, uxtw]

[steplong]

• Change addrspace(256) to the default addrspace 0

[steplong]

• Use CharUnits::One() instead of getTypeAlignInChars()

[efriedma]

LGTM

[steplong]

• Rebased patch on top of main