Driver: correct the registrar extension

The object file extension on Windows is `.obj` rather than `.o`.  Ensure
that we get the extension correct when compiling for Windows.
Furthermore, ensure that we install with the correct extension when
installing a cross-compiled image.  As we may be using the homegrown
cross-compilation system, we must explicitly handle the extension
ourselves.  This allows nearly a 100 additional tests to pass on
Windows.

Author: compnerd

lib/Driver/WindowsToolChains.cpp

@@ -122,7 +122,7 @@ toolchains::Windows::constructInvocation(const LinkJobAction &job,
   SmallString<128> swiftrtPath = SharedRuntimeLibPath;
   llvm::sys::path::append(swiftrtPath,
                           swift::getMajorArchitectureName(getTriple()));
-  llvm::sys::path::append(swiftrtPath, "swiftrt.o");
+  llvm::sys::path::append(swiftrtPath, "swiftrt.obj");
   Arguments.push_back(context.Args.MakeArgString(swiftrtPath));
 
   addPrimaryInputsOfType(Arguments, context.Inputs, context.Args,

stdlib/public/runtime/CMakeLists.txt

@@ -216,8 +216,15 @@ foreach(sdk ${SWIFT_CONFIGURED_SDKS})
       # to a version which supports it.
       # set(swiftrtObject "$<TARGET_OBJECTS:swiftImageRegistrationObject${SWIFT_SDK_${sdk}_OBJECT_FORMAT}-${arch_suffix}>")
       set(swiftrtObject ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/swiftImageRegistrationObject${SWIFT_SDK_${sdk}_OBJECT_FORMAT}-${arch_suffix}.dir/SwiftRT-${SWIFT_SDK_${sdk}_OBJECT_FORMAT}.cpp${CMAKE_C_OUTPUT_EXTENSION})
-      set(shared_runtime_registrar "${SWIFTLIB_DIR}/${arch_subdir}/swiftrt${CMAKE_C_OUTPUT_EXTENSION}")
-      set(static_runtime_registrar "${SWIFTSTATICLIB_DIR}/${arch_subdir}/swiftrt${CMAKE_C_OUTPUT_EXTENSION}")
+
+      if(sdk STREQUAL WINDOWS)
+        set(extension .obj)
+      else()
+        set(extension .o)
+      endif()
+
+      set(shared_runtime_registrar "${SWIFTLIB_DIR}/${arch_subdir}/swiftrt${extension}")
+      set(static_runtime_registrar "${SWIFTSTATICLIB_DIR}/${arch_subdir}/swiftrt${extension}")
 
       add_custom_command_target(swiftImageRegistration-${arch_suffix}
                                 COMMAND