[CodeGen] Correctly handle base classes which are passed in memory

[majnemer]

Summary

We didn't correctly process the case where a base class is classified as
MEMORY. This would cause us to trip over an assertion.

This fixes PR24020.

[rnk]

lgtm

lib/CodeGen/TargetInfo.cpp

if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
         // single eightbyte, each is classified separately. Each eightbyte gets
         // initialized to class NO_CLASS.
         Class FieldLo, FieldHi;
         uint64_t Offset =
           OffsetBase + getContext().toBits(Layout.getBaseClassOffset(Base));
         classify(I.getType(), Offset, FieldLo, FieldHi, isNamedArg);
         Lo = merge(Lo, FieldLo);
         Hi = merge(Hi, FieldHi);
-        if (Lo == Memory || Hi == Memory)
-          break;
+        if (Lo == Memory || Hi == Memory) {
+          postMerge(Size, Lo, Hi);
+          return;
+        }
       }
     }
 
     // Classify the fields one at a time, merging the results.
     unsigned idx = 0;
     for (RecordDecl::field_iterator i = RD->field_begin(), e = RD->field_end();
            i != e; ++i, ++idx) {
       uint64_t Offset = OffsetBase + Layout.getFieldOffset(idx);

test/CodeGenCXX/x86_64-arguments.cpp

S c(int, int, int, T, void*) {
     return S();
   }
 
   // CHECK: define [[S]]* @_ZN5test91dEPNS_1SEiiiNS_1TEPv([[S]]* {{%.*}}, i32, i32, i32, i8* {{%.*}}, i8* {{%.*}}, i8*)
   S* d(S* sret, int, int, int, T, void*) {
     return sret;
   }
 }
+
+namespace test10 {
+#pragma pack(1)
+struct BasePacked {
+  char one;
+  short two;
+};
+#pragma pack()
+struct DerivedPacked : public BasePacked {
+  int three;
+};
+// CHECK-LABEL: define i32 @_ZN6test1020FuncForDerivedPackedENS_13DerivedPackedE({{.*}}* byval align 8
+int FuncForDerivedPacked(DerivedPacked d) {
+  return d.three;
+}
+}