[clang] Fix -Wunused-variable in InterpBuiltin.cpp (NFC)

llvm-project/clang/lib/AST/Interp/InterpBuiltin.cpp:1332:21:
error: unused variable 'SrcDesc' [-Werror,-Wunused-variable]
  const Descriptor *SrcDesc = Src.getFieldDesc();
                    ^
1 error generated.