Set the target linker language for the flang libraries to CXX on windows.

This makes sure link.exe from Visual Studio is used to link the libraries, even though the libraries include fortran code.
CMake is not able to detect this since the flang fortran compiler is not working at CMake configure time and thus the CMAKE_Fortran_COMPILER_ID is unknown.

Author: Albert Ziegenhagel

runtime/flang/CMakeLists.txt

@@ -579,6 +579,9 @@ add_dependencies(flang_shared
 if (NOT MSVC)
   target_compile_options(flang_static PRIVATE -fPIC)
   target_compile_options(flang_shared PRIVATE -fPIC)
+else()
+  set_target_properties(flang_static PROPERTIES LINKER_LANGUAGE CXX)
+  set_target_properties(flang_shared PROPERTIES LINKER_LANGUAGE CXX)
 endif()
 
 target_compile_options(flang_static PUBLIC $<$<COMPILE_LANGUAGE:Fortran>:-Mreentrant>)