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

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.

Patch by Chris January

Differential Revision: https://reviews.llvm.org/D44087

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@326744 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang