In the dev12 PDB format, usings and imports were encoded as strings in
PDB files.  The strings were structured, but required a certain amount of
guesswork (for example, you might not know whether you were importing a
type or a namespace until after binding).  In the new PDB format, usings
and imports are represented as structured blobs and tokens are used
wherever possible.

This change introduces two new class hierarchies, ImportRecord and
ExternAliasRecord, that the expression compiler can pass around instead
of raw strings.  Unfortunately, there is still switching logic in two
places - both when these types are instantiated and when they are
consumed.  The instantiation switch is very simple - we just check for
the appropriate version of ISymUnmanagedReaderX.  The consumption logic
is more complex, because the new and old representations differ
substantially.

TODO: Presently, there is very little code for consuming the new PDB
format, since the APIs are not yet available.