GC plugin: split out reporting of errors/warnings.

tools/clang/blink_gc_plugin/CMakeLists.txt

 set(LIBRARYNAME BlinkGCPlugin)
 
 set(plugin_sources
   BlinkGCPlugin.cpp
   BlinkGCPluginConsumer.cpp
   CheckDispatchVisitor.cpp
   CheckFieldsVisitor.cpp
   CheckFinalizerVisitor.cpp
   CheckGCRootsVisitor.cpp
   CheckTraceVisitor.cpp
   CollectVisitor.cpp
   Config.cpp
+  DiagnosticsReporter.cpp
   Edge.cpp
   RecordInfo.cpp)
 
 if(WIN32)
   # Clang doesn't support loadable modules on Windows. Unfortunately, building
   # the plugin as a static library and linking clang against it doesn't work.
   # Since clang doesn't reference any symbols in our static library, the linker
   # strips it out completely.
   # Instead, we rely on the fact that the SOURCES property of a target is no
   # read-only after CMake 3.1 and use it to compile the plugin directly into
@@ skipping 15 matching lines @@
   add_dependencies("lib${LIBRARYNAME}" clang)
 
   cr_install(TARGETS "lib${LIBRARYNAME}" LIBRARY DESTINATION lib)
 
   cr_add_test(blink_gc_plugin_test
     python tests/test.py
     ${CMAKE_BINARY_DIR}/bin/clang
     $<TARGET_FILE:lib${LIBRARYNAME}>
     )
 endif()