[CMake] Make sure to link swiftrt.o when building the Swift Dispatch overlay.

Because we are linking libdispatch through clang rather than swift, we need to
explicitly add swiftrt.o, which is needed by ELF and COFF to register metadata
sections. Fixes rdar://problem/44941707.

Author: DougGregor

src/CMakeLists.txt

@@ -115,7 +115,8 @@ if(ENABLE_SWIFT)
   target_sources(dispatch
                  PRIVATE
                    swift/DispatchStubs.cc
-                   ${CMAKE_CURRENT_BINARY_DIR}/swiftDispatch.o)
+                   ${CMAKE_CURRENT_BINARY_DIR}/swiftDispatch.o
+                   ${SWIFT_RUNTIME_LIBDIR}/swiftrt.o)
   if(CMAKE_BUILD_TYPE MATCHES Debug)
     target_link_libraries(dispatch
                           PRIVATE