[modules] Store the location of the lexical update record in a decl update as
an offset from the current record rather than as an absolute bit number. This
gives a minor .pcm file size reduction.

llvm-svn: 266269
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 2fec5c2..5b5ecb1 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -4661,8 +4661,7 @@
         auto *RD = cast<CXXRecordDecl>(D);
         UpdatedDeclContexts.insert(RD->getPrimaryContext());
         Record.AddCXXDefinitionData(RD);
-        // FIXME: Use AddOffset here.
-        Record.push_back(WriteDeclContextLexicalBlock(
+        Record.AddOffset(WriteDeclContextLexicalBlock(
             *Context, const_cast<CXXRecordDecl *>(RD)));
 
         // This state is sometimes updated by template instantiation, when we