build: be more clever about path to clang

Setup the clang build to use clang-cl on Windows and clang on other
targets.  This allows libdispatch to build on windows when building with
cl.  libdispatch requires clang-cl to build as it relies on clang
features.

Author: compnerd

CMakeLists.txt

@@ -909,8 +909,18 @@ if(SWIFT_NEED_EXPLICIT_LIBDISPATCH)
     set(SWIFT_LIBDISPATCH_C_COMPILER ${CMAKE_C_COMPILER})
     set(SWIFT_LIBDISPATCH_CXX_COMPILER ${CMAKE_CXX_COMPILER})
   elseif(${CMAKE_SYSTEM_NAME} STREQUAL ${CMAKE_HOST_SYSTEM_NAME})
-    set(SWIFT_LIBDISPATCH_C_COMPILER $<TARGET_PROPERTY:clang,LOCATION>)
-    set(SWIFT_LIBDISPATCH_CXX_COMPILER $<TARGET_PROPERTY:clang,LOCATION>)
+    get_target_property(CLANG_LOCATION clang LOCATION)
+    get_filename_component(CLANG_LOCATION ${CLANG_LOCATION} DIRECTORY)
+
+    if(CMAKE_SYSTEM_NAME STREQUAL Windows)
+      set(SWIFT_LIBDISPATCH_C_COMPILER
+          ${CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+      set(SWIFT_LIBDISPATCH_CXX_COMPILER
+          ${CLANG_LOCATION}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+    else()
+      set(SWIFT_LIBDISPATCH_C_COMPILER ${CLANG_LOCATION}/clang)
+      set(SWIFT_LIBDISPATCH_CXX_COMPILER ${CLANG_LOCATION}/clang++)
+    endif()
   else()
     message(SEND_ERROR "libdispatch requires a newer clang compiler (${CMAKE_C_COMPILER_VERSION} < 3.9)")
   endif()