[Swift/C++ interop] Find Swiftrt file

The Windows toolchains keep the Swift runtime in the SDK instead of in
the toolchain. The driver lists the runtime locations for the toolchain,
but doesn't list them for the SDK, so we don't really have a great way
to ask for the location. Instead, we translate the system processor to
the name of the architecture directory in the swift SDK and grab the
file directly.

Author: etcwilde

3_bidirectional_cxx_interop/cmake/modules/InitializeSwift.cmake

@@ -5,6 +5,22 @@
 #
 # See https://swift.org/LICENSE.txt for license information
 
+# Compute the name of the architecture directory on Windows from the CMake
+# system processor name.
+function(_swift_windows_arch_name output_variable_name target_arch)
+  if(NOT WIN32)
+    return()
+  endif()
+
+  if("${target_arch}" STREQUAL "AMD64")
+    set("${output_variable_name}" "x86_64" PARENT_SCOPE)
+  elseif("${target_arch}" STREQUAL "ARM64")
+    set("${output_variable_name}" "aarch64" PARENT_SCOPE)
+  else()
+    message(FATAL_ERROR "Unknown windows architecture: ${target_arch}")
+  endif()
+endfunction()
+
 # Compute flags and search paths
 # NOTE: This logic will eventually move to CMake
 function(_setup_swift_paths)
@@ -55,7 +71,11 @@ function(_setup_swift_paths)
 
   link_directories(${SWIFT_LIBRARY_SEARCH_PATHS})
 
-  if(NOT APPLE)
+  if(WIN32)
+    _swift_windows_arch_name(SWIFT_WIN_ARCH_DIR "${CMAKE_SYSTEM_PROCESSOR}")
+    set(SWIFT_SWIFTRT_FILE "$ENV{SDKROOT}/usr/lib/swift/windows/${SWIFT_WIN_ARCH_DIR}/swiftrt.obj")
+    add_link_options("$<$<LINK_LANGUAGE:Swift>:${SWIFT_SWIFTRT_FILE}>")
+  elseif(NOT APPLE)
     find_file(SWIFT_SWIFTRT_FILE
               swiftrt.o
               PATHS ${SWIFT_LIBRARY_SEARCH_PATHS}