[cxx-interop] Fix IRGen for C++ types that use tail padding of their bases

In C++, a field of a derived class might be placed into the tail padding of a base class. Swift was not handling this case correctly, causing an asserts-disabled compiler to run out of RAM, and an asserts-enabled compiler to fail with an assertion.

Fixes this IRGen assertion:
```
Assertion failed: (offset >= NextOffset && "adding fields out of order"), function addField, file GenStruct.cpp, line 1509.
```

rdar://138764929

lib/IRGen/GenStruct.cpp

@@ -1423,7 +1423,8 @@ private:
     auto sfi = swiftProperties.begin(), sfe = swiftProperties.end();
     // When collecting fields from the base subobjects, we do not have corresponding swift
     // stored properties.
-    if (decl != ClangDecl)
+    bool isBaseSubobject = decl != ClangDecl;
+    if (isBaseSubobject)
       sfi = swiftProperties.end();
 
     while (cfi != cfe) {
@@ -1475,10 +1476,14 @@ private:
 
     assert(sfi == sfe && "more Swift fields than there were Clang fields?");
 
-    Size objectTotalStride = Size(layout.getSize().getQuantity());
+    auto objectSize = isBaseSubobject ? layout.getDataSize() : layout.getSize();
-    // We never take advantage of tail padding, because that would prevent
+    Size objectTotalStride = Size(objectSize.getQuantity());
-    // us from passing the address of the object off to C, which is a pretty
+    // Unless this is a base subobject of a C++ type, we do not take advantage
-    // likely scenario for imported C types.
+    // of tail padding, because that would prevent us from passing the address
+    // of the object off to C, which is a pretty likely scenario for imported C
+    // types.
+    // In C++, fields of a derived class might get placed into tail padding of a
+    // base class, in which case we should not add extra padding here.
     assert(NextOffset <= SubobjectAdjustment + objectTotalStride);
     assert(SpareBits.size() <= SubobjectAdjustment.getValueInBits() +
                                    objectTotalStride.getValueInBits());

test/Interop/Cxx/class/inheritance/Inputs/fields.h

@@ -118,3 +118,17 @@ struct InheritFromStructsWithVirtualMethod: HasOneFieldWithVirtualMethod, HasTwo
   int d;
   virtual ~InheritFromStructsWithVirtualMethod() = default;
 };
+
+// MARK: Types that pack their fields into tail padding of a base class.
+
+struct BaseAlign8 {
+  long long field8 = 123;
+}; // sizeof=8, dsize=8, align=8
+
+struct DerivedHasTailPadding : public BaseAlign8 {
+  int field4 = 456;
+}; // sizeof=16, dsize=12, align=8
+
+struct DerivedUsesBaseTailPadding : public DerivedHasTailPadding {
+  short field2 = 789;
+}; // sizeof=16, dsize=14, align=8

test/Interop/Cxx/class/inheritance/fields.swift

@@ -117,4 +117,11 @@ FieldsTestSuite.test("Structs with virtual methods") {
   expectEqual(derived.d, 42)
 }
 
+FieldsTestSuite.test("Field in tail padding of base class") {
+  let usesBaseTailPadding = DerivedUsesBaseTailPadding()
+  expectEqual(usesBaseTailPadding.field2, 789)
+  expectEqual(usesBaseTailPadding.field4, 456)
+  expectEqual(usesBaseTailPadding.field8, 123)
+}
+
 runAllTests()