AsmWriter: Assert on unresolved metadata nodes
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.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@232442 91177308-0d34-0410-b5e6-96231b3b80d8

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