[CodeGenObjC] Fix a crash when attempting to copy a zero-sized bit-field in a non-trivial C struct

Zero sized bit-fields aren't included in the CGRecordLayout, so we shouldn't be
calling EmitLValueForField for them. rdar://60695105

Differential revision: https://reviews.llvm.org/D76782
diff --git a/clang/test/CodeGenObjC/strong-in-c-struct.m b/clang/test/CodeGenObjC/strong-in-c-struct.m
index ec212c4..f022711 100644
--- a/clang/test/CodeGenObjC/strong-in-c-struct.m
+++ b/clang/test/CodeGenObjC/strong-in-c-struct.m
@@ -887,4 +887,17 @@
   func(0);
 }
 
+struct ZeroBitfield {
+  int : 0;
+  id strong;
+};
+
+
+// CHECK: define linkonce_odr hidden void @__default_constructor_8_sv0
+// CHECK: define linkonce_odr hidden void @__copy_assignment_8_8_sv0
+void test_zero_bitfield() {
+  struct ZeroBitfield volatile a, b;
+  a = b;
+}
+
 #endif /* USESTRUCT */