[clang] Implement [[msvc::no_unique_address]] (PR #65675)

[Amy Huang via cfe-commits]

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@@ -2937,14 +2964,97 @@ void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
       BaseOffset = CharUnits::Zero();
     } else {
       // Otherwise, lay the base out at the end of the MDC.
-      BaseOffset = Size = Size.alignTo(Info.Alignment);
+      BaseOffset = DataSize = Size = Size.alignTo(Info.Alignment);
     }
+
+    // Place in EmptySubobjects map but don't check the position? MSVC seems to
+    // not allow fields to overlap at the end of a virtual base, but they can
+    // overlap with other bass.
+    EmptySubobjects->CanPlaceBaseAtOffset(Base, BaseOffset);
   }
+
   Bases.insert(std::make_pair(BaseDecl, BaseOffset));
   Size += BaseLayout.getNonVirtualSize();
+  DataSize = Size;
   PreviousBaseLayout = &BaseLayout;
 }
 
+BaseSubobjectInfo *MicrosoftRecordLayoutBuilder::computeBaseSubobjectInfo(
+    const CXXRecordDecl *RD, bool IsVirtual, BaseSubobjectInfo *Derived) {
+  // This is copied directly from ItaniumRecordLayoutBuilder::ComputeBaseSubobjectInfo.
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amykhuang wrote:

Actually going to remove this altogether. Makes it less consistent with MSVC's behavior but MSVC appears to avoid overlapping fields with bases only when no_unique_address is used, which seems weird. 

https://github.com/llvm/llvm-project/pull/65675