BlinkGCPlugin: Support overloaded trace() call from template.

tools/clang/blink_gc_plugin/tests/traceimpl_overloaded_error.txt

+In file included from traceimpl_overloaded_error.cpp:5:
+./traceimpl_overloaded_error.h:27:3: warning: [blink-gc] Class 'InlinedBase' has untraced fields that require tracing.
+  void traceImpl(VisitorDispatcher visitor) {
+  ^
+./traceimpl_overloaded_error.h:31:3: note: [blink-gc] Untraced field 'x_base_' declared here:
+  Member<X> x_base_;
+  ^
+./traceimpl_overloaded_error.h:43:3: warning: [blink-gc] Base class 'InlinedBase' of derived class 'InlinedDerived' requires tracing.
+  void traceImpl(VisitorDispatcher visitor) {
+  ^
+./traceimpl_overloaded_error.h:43:3: warning: [blink-gc] Class 'InlinedDerived' has untraced fields that require tracing.
+./traceimpl_overloaded_error.h:47:3: note: [blink-gc] Untraced field 'x_derived_' declared here:
+  Member<X> x_derived_;
+  ^
+traceimpl_overloaded_error.cpp:18:1: warning: [blink-gc] Class 'ExternBase' has untraced fields that require tracing.
+inline void ExternBase::traceImpl(VisitorDispatcher visitor) {
+^
+./traceimpl_overloaded_error.h:59:3: note: [blink-gc] Untraced field 'x_base_' declared here:
+  Member<X> x_base_;
+  ^
+traceimpl_overloaded_error.cpp:31:1: warning: [blink-gc] Base class 'ExternBase' of derived class 'ExternDerived' requires tracing.
+inline void ExternDerived::traceImpl(VisitorDispatcher visitor) {
+^
+traceimpl_overloaded_error.cpp:31:1: warning: [blink-gc] Class 'ExternDerived' has untraced fields that require tracing.
+./traceimpl_overloaded_error.h:71:3: note: [blink-gc] Untraced field 'x_derived_' declared here:
+  Member<X> x_derived_;
+  ^
+6 warnings generated.