[IndexStore] Keep indexstore_symbol_role_t enum bits fixed
They were previously required to exactly match the SymbolRole enum, and shifted
when a new Undefined role was added there. The indexstore values were intended
to be stable though, so this patch implements mapping functions between the two,
and restores the indexstore bits to their original values, assigning 'undefined'
the next available bit instead of matching its value in SymbolRole.
Also fixes a few places where the conversion functions weren't being called.