Add checks for stack-allocated types and their uses in fields.

tools/clang/blink_gc_plugin/tests/stack_allocated.txt

Index: tools/clang/blink_gc_plugin/tests/stack_allocated.txt
diff --git a/tools/clang/blink_gc_plugin/tests/stack_allocated.txt b/tools/clang/blink_gc_plugin/tests/stack_allocated.txt
index f5745bb7364b9e02ff87dbfbe8fac98820371ada..acdb481fb83120be24fccfeab86a4e8f700324e7 100644
--- a/tools/clang/blink_gc_plugin/tests/stack_allocated.txt
+++ b/tools/clang/blink_gc_plugin/tests/stack_allocated.txt
@@ -5,4 +5,16 @@ class PartObject {
 ./stack_allocated.h:17:5: note: [blink-gc] Untraced field 'm_obj' declared here:
     Member<HeapObject> m_obj; // Needs tracing.
     ^
-1 warning generated.
+./stack_allocated.h:26:28: warning: [blink-gc] Stack-allocated class 'AnotherStackObject' derives class 'PartObject' which is not stack allocated.
+class AnotherStackObject : public PartObject { // Invalid base.
+                           ^
+./stack_allocated.h:32:1: warning: [blink-gc] Class 'HeapObject' contains invalid fields.
+class HeapObject : public GarbageCollected<HeapObject> {
+^
+./stack_allocated.h:36:5: note: [blink-gc] Stack-allocated field 'm_part' declared here:
+    StackObject m_part; // Cannot embed a stack allocated object.
+    ^
+./stack_allocated.h:39:28: warning: [blink-gc] Class 'DerivedStackObject' derives stack-allocated class 'StackObject' but is not itself stack allocated
+class DerivedStackObject : public StackObject {
+                           ^
+4 warnings generated.