Driver: rename Windows to MSVCToolChain

This renames the Windows toolchain to MSVCToolChain.  This is a preparatory step
for adding a CrossWindowsToolChain which uses clang/libc++/(ld/lld) without the
standard GCC toolchain lookup.  NFC.

llvm-svn: 220362
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index ccefbd8..9fea2f4 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2041,7 +2041,7 @@
         break;
       case llvm::Triple::MSVC:
       case llvm::Triple::UnknownEnvironment:
-        TC = new toolchains::Windows(*this, Target, Args);
+        TC = new toolchains::MSVCToolChain(*this, Target, Args);
         break;
       }
       break;