[clang] [lldb] [clang][RecordLayoutBuilder] Be stricter about inferring packed-ness in ExternalLayouts (PR #97443)

[John McCall via cfe-commits]

================
@@ -2243,9 +2236,27 @@ ItaniumRecordLayoutBuilder::updateExternalFieldOffset(const FieldDecl *Field,
                                                       uint64_t ComputedOffset) {
   uint64_t ExternalFieldOffset = External.getExternalFieldOffset(Field);
 
-  if (InferAlignment && ExternalFieldOffset < ComputedOffset) {
-    // The externally-supplied field offset is before the field offset we
-    // computed. Assume that the structure is packed.
+  // DWARF doesn't tell us whether a structure was declared as packed.
+  // So we try to figure out if the supplied Field is at a packed offset
+  // (i.e., the externally-supplied offset is less than the layout builder
+  // expected).
+  //
+  // There are cases where fields are placed at overlapping offsets (e.g.,
+  // as a result of [[no_unique_address]]). In those cases we don't want
+  // to incorrectly deduce that they are placed at packed offsets. Hence,
+  // ignore empty fields (which are the only fields that can overlap).
+  //
+  // FIXME: emit enough information in DWARF to get rid of InferAlignment.
+  //
+  CXXRecordDecl *CXX = nullptr;
+  if (auto *RT = dyn_cast<RecordType>(Field->getType()))
+    CXX = RT->getAsCXXRecordDecl();
----------------
rjmccall wrote:

This is all just `auto CXX = Field->getType()->getAsCXXRecordDecl();`

https://github.com/llvm/llvm-project/pull/97443