rustc: Better error when loading invalid libraries

When the metadata format changes, old libraries often cause librustc to abort
when reading their metadata. This should all change with the introduction of SVH
markers, but the loader for crates should gracefully handle libraries without
SVH markers still.

This commit adds support for tripping fewer assertions when loading libraries by
using maybe_get_doc when initially parsing metadata. It's still possible for
some libraries to fall through the cracks, but this should deal with a fairly
large number of them up front.

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