Support roundtripping symbols with unknown text.

[tgregg]

This is possible now that SymbolToken tracks ImportLocation. Two bits of logic are required:

1. When the reader encounters a symbol ID that occurs within a shared symbol table but that shared symbol table is not resolved in the catalog, the reader should return a SymbolToken with null text and an ImportLocation that identifies the name and SID in the unresolved shared symbol table.
2. When the writer is provided with a SymbolToken with null text and with a valid ImportLocation, it should calculate the local symbol ID from the import symbol ID and write the local symbol ID.