[msvc] Allow MSVC toolchain driver to find the aarch64 / arm64 cross-compiler.

[arm-chrjan01]

Summary

Starting with the Fall Creators Update, Windows 10 Desktop can run on machines that are powered by aarch64 processors.
Microsoft call the aarch64 architecture "arm64". This patch maps ArchType::aarch64 to "arm64" to allow the MSVC toolchain driver to find the aarch64 / arm64 cross-compiler.

[arm-chrjan01]

I don't have commit access. Please commit after approval.

[rnk]

lgtm

cfe/trunk/lib/Driver/ToolChains/MSVC.cpp

static const char *llvmArchToWindowsSDKArch(llvm::Triple::ArchType Arch) {
   using ArchType = llvm::Triple::ArchType;
   switch (Arch) {
   case ArchType::x86:
     return "x86";
   case ArchType::x86_64:
     return "x64";
   case ArchType::arm:
     return "arm";
+  case ArchType::aarch64:
+    return "arm64";
   default:
     return "";
   }
 }
 
 // Similar to the above function, but for Visual Studios before VS2017.
 static const char *llvmArchToLegacyVCArch(llvm::Triple::ArchType Arch) {
   using ArchType = llvm::Triple::ArchType;
   switch (Arch) {
   case ArchType::x86:
     // x86 is default in legacy VC toolchains.
     // e.g. x86 libs are directly in /lib as opposed to /lib/x86.
     return "";
   case ArchType::x86_64:
     return "amd64";
   case ArchType::arm:
     return "arm";
+  case ArchType::aarch64:
+    return "arm64";
   default:
     return "";
   }
 }
 
 // Similar to the above function, but for DevDiv internal builds.
 static const char *llvmArchToDevDivInternalArch(llvm::Triple::ArchType Arch) {
   using ArchType = llvm::Triple::ArchType;
   switch (Arch) {
   case ArchType::x86:
     return "i386";
   case ArchType::x86_64:
     return "amd64";
   case ArchType::arm:
     return "arm";
+  case ArchType::aarch64:
+    return "arm64";
   default:
     return "";
   }
 }
 
 // Get the path to a specific subdirectory in the current toolchain for
 // a given target architecture.
 // VS2017 changed the VC toolchain layout, so this should be used instead