[NFC] Assert that extra data size matches extra data pattern and offset.

To prevent the fix for rdar://problem/68997282 from regressing, assert
that the size of the extra data patterns for generic enums and structs
combined with the corresponding offsets matches the size passed from the
generic metadata instantiation function to
swift_allocateGenericValueMetadata.

Author: nate-chandler

lib/IRGen/GenMeta.cpp

@@ -4160,13 +4160,18 @@ namespace {
                                  ConstantStructBuilder &B)
       : super(IGM, theStruct, B) {}
 
+    Size getExtraDataSize(StructMetadataLayout &layout) {
+      auto extraSize = layout.getSize().getOffsetToEnd() -
+                       IGM.getOffsetOfStructTypeSpecificMetadataMembers();
+      return extraSize;
+    }
+
     llvm::Value *emitAllocateMetadata(IRGenFunction &IGF,
                                       llvm::Value *descriptor,
                                       llvm::Value *arguments,
                                       llvm::Value *templatePointer) {
       auto &layout = IGM.getMetadataLayout(Target);
-      auto extraSize = layout.getSize().getOffsetToEnd()
-                         - IGM.getOffsetOfStructTypeSpecificMetadataMembers();
+      auto extraSize = getExtraDataSize(layout);
       auto extraSizeV = IGM.getSize(extraSize);
 
       // Sign the descriptor.
@@ -4279,10 +4284,9 @@ namespace {
       Offset zeroingEnd = offsetUpToTrailingFlags 
                             ? layout.getTrailingFlagsOffset()
                             : layout.getFieldOffsetVectorOffset();
-      return { global,
-               zeroingEnd.getStatic()
-                 - zeroingStart,
-               structSize };
+      auto offset = zeroingEnd.getStatic() - zeroingStart;
+      assert((offset + structSize) == getExtraDataSize(layout));
+      return {global, offset, structSize};
     }
 
     bool hasCompletionFunction() {
@@ -4558,13 +4562,18 @@ namespace {
                                ConstantStructBuilder &B)
       : super(IGM, theEnum, B) {}
 
+    Size getExtraDataSize(EnumMetadataLayout &layout) {
+      auto size = layout.getSize().getOffsetToEnd() -
+                  IGM.getOffsetOfEnumTypeSpecificMetadataMembers();
+      return size;
+    }
+
     llvm::Value *emitAllocateMetadata(IRGenFunction &IGF,
                                       llvm::Value *descriptor,
                                       llvm::Value *arguments,
                                       llvm::Value *templatePointer) {
       auto &layout = IGM.getMetadataLayout(Target);
-      auto extraSize = layout.getSize().getOffsetToEnd()
-                         - IGM.getOffsetOfEnumTypeSpecificMetadataMembers();
+      auto extraSize = getExtraDataSize(layout);
       auto extraSizeV = IGM.getSize(extraSize);
 
       // Sign the descriptor.
@@ -4625,10 +4634,9 @@ namespace {
       Offset zeroingEnd = offsetUpToTrailingFlags 
                             ? layout.getTrailingFlagsOffset()
                             : layout.getPayloadSizeOffset();
-      return { global,
-               zeroingEnd.getStatic()
-                 - zeroingStart,
-               structSize };
+      auto offset = zeroingEnd.getStatic() - zeroingStart;
+      assert((offset + structSize) == getExtraDataSize(layout));
+      return {global, offset, structSize};
     }
 
     llvm::Constant *emitNominalTypeDescriptor() {