AsmWriter: Assert on unresolved metadata nodes

Assert that `MDNode::isResolved()`.  While in theory the `Verifier`
should catch this, it doesn't descend into all debug info, and the
`DebugInfoVerifier` doesn't call into the `Verifier`.  Besides, this
helps to catch bugs when `-disable-verify=true`.

Note that I haven't come across a place where this fails with clang
today, so no testcase.

llvm-svn: 232442