Revert "[clang] Remove unused variable 'ExprDependenceBits' in ASTWriterDecl.cpp (NFC)"

This reverts commit 1095105.
This should be part of 8c33462 to revert
9406ea3 completely.

Author: ChuanqiXu9

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -2346,6 +2346,7 @@ void ASTWriter::WriteDeclAbbrevs() {
   Abv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));
   DeclCXXMethodAbbrev = Stream.EmitAbbrev(std::move(Abv));
 
+  unsigned ExprDependenceBits = llvm::BitWidth<ExprDependence>;
   // Abbreviation for EXPR_DECL_REF
   Abv = std::make_shared<BitCodeAbbrev>();
   Abv->Add(BitCodeAbbrevOp(serialization::EXPR_DECL_REF));