[cmake] Force the linker_language of host executables to be CXX.

In certain cases our executables are going to have a mix of cxx,swift. There are
current issues when linking cxx,swift code with swiftc itself, so this lets us
avoid that problem.

Author: gottesmm

cmake/modules/AddSwift.cmake

@@ -572,6 +572,9 @@ function(add_swift_host_tool executable)
   _add_host_variant_c_compile_link_flags(${executable})
   target_link_directories(${executable} PRIVATE
     ${SWIFTLIB_DIR}/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_LIB_SUBDIR})
+  # Force executables linker language to be CXX so that we do not link using the
+  # host toolchain swiftc.
+  set_target_properties(${executable} PROPERTIES LINKER_LANGUAGE CXX)
   add_dependencies(${executable} ${LLVM_COMMON_DEPENDS})
 
   set_target_properties(${executable} PROPERTIES