Fix compilation issue reported by MSVC user on cfe-dev

MSVC seems to think this `friend class TrailingObjects;` declaration is
declaring a TrailingObjects class instead of naming the injected base
class. Remove `class` so it does the right thing.

clang/include/clang/AST/ExprCXX.h

@@ -4864,7 +4864,7 @@ class BuiltinBitCastExpr final
       private llvm::TrailingObjects<BuiltinBitCastExpr, CXXBaseSpecifier *> {
   friend class ASTStmtReader;
   friend class CastExpr;
-  friend class TrailingObjects;
+  friend TrailingObjects;
 
   SourceLocation KWLoc;
   SourceLocation RParenLoc;