[clang] Add ARM64 support to armintr.h for MSVC compatibility

[mgrang]

Summary

This fixes compiling with headers from the Windows SDK for ARM64.

[mgrang]

Refer http://infocenter.arm.com/help/topic/com.arm.doc.ihi0053c/IHI0053C_acle_2_0.pdf for memory barriers.

[mstorsjo]

The Microsoft version of intrin.h includes a header named arm64intr.h (IIRC), I think it'd be better for compatibility if we also provided a file with the same name instead of reusing this one.

[compnerd]

Please split this out into arm64intr.h and include it from intrin.h under a #if defined(_M_ARM64) case before committing.

[mgrang]

Patch updated as per comments.

lib/Headers/armintr.h

typedef enum
   _ARM_BARRIER_ISH   = 0xB,
   _ARM_BARRIER_ISHST = 0xA,
   _ARM_BARRIER_NSH   = 0x7,
   _ARM_BARRIER_NSHST = 0x6,
   _ARM_BARRIER_OSH   = 0x3,
   _ARM_BARRIER_OSHST = 0x2
 } _ARMINTR_BARRIER_TYPE;
 
+typedef enum
+{
+  _ARM64_BARRIER_SY    = 0xF,
+  _ARM64_BARRIER_ST    = 0xE,
+  _ARM64_BARRIER_LD    = 0xD,
+  _ARM64_BARRIER_ISH   = 0xB,
+  _ARM64_BARRIER_ISHST = 0xA,
+  _ARM64_BARRIER_ISHLD = 0x9,
+  _ARM64_BARRIER_NSH   = 0x7,
+  _ARM64_BARRIER_NSHST = 0x6,
+  _ARM64_BARRIER_NSHLD = 0x5,
+  _ARM64_BARRIER_OSH   = 0x3,
+  _ARM64_BARRIER_OSHST = 0x2,
+  _ARM64_BARRIER_OSHLD = 0x1
+} _ARM64INTR_BARRIER_TYPE;
+
 #endif /* __ARMINTR_H */
 #endif /* _MSC_VER */