Use an overload instead of a default argument for `Metadata::dump()`.
The latter seems to require calling `dump(nullptr)` explicitly when
using a debugger, where as the former doesn't.
Other than utility for debugging, there's NFC here.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232315 91177308-0d34-0410-b5e6-96231b3b80d8