build: enable cross-compilation on Windows

When cross-compiling the toolchain for Windows, we would try to run the
foreign target compiler on the build which does not run.  Fallback to
the path to find a `clang-cl` that we can use.

Author: compnerd

CMakeLists.txt

@@ -953,10 +953,15 @@ if(SWIFT_BUILD_SYNTAXPARSERLIB OR SWIFT_BUILD_SOURCEKIT)
       set(SWIFT_LIBDISPATCH_CXX_COMPILER ${CMAKE_CXX_COMPILER})
     elseif(${CMAKE_SYSTEM_NAME} STREQUAL ${CMAKE_HOST_SYSTEM_NAME})
       if(CMAKE_SYSTEM_NAME STREQUAL Windows)
-        set(SWIFT_LIBDISPATCH_C_COMPILER
-            $<TARGET_FILE_DIR:clang>/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
-        set(SWIFT_LIBDISPATCH_CXX_COMPILER
-            $<TARGET_FILE_DIR:clang>/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+        if(TARGET clang)
+          set(SWIFT_LIBDISPATCH_C_COMPILER
+              $<TARGET_FILE_DIR:clang>/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+          set(SWIFT_LIBDISPATCH_CXX_COMPILER
+              $<TARGET_FILE_DIR:clang>/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+        else()
+          set(SWIFT_LIBDISPATCH_C_COMPILER clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+          set(SWIFT_LIBDISPATCH_CXX_COMPILER clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+        endif()
       else()
         set(SWIFT_LIBDISPATCH_C_COMPILER $<TARGET_FILE_DIR:clang>/clang)
         set(SWIFT_LIBDISPATCH_CXX_COMPILER $<TARGET_FILE_DIR:clang>/clang++)