1. Use ConstructedFrom instead of OriginalDefinition to handle partially
   constructed types.
2. Always encode type parameter references using `n or ``n, since they can
   only refer to type parameters within the current symbol being encoded.
3. When *parsing* SymbolIds, *do* set the type parameter context when
   parsing types.  That way we can handle self-constructed types.  However,
   we don't want to reset the context when we're parsing out the type of a
   parameter/return type/etc, because the signature *can't* refer to the
   type parameters of *that* simple.  So, now we keep track of when the
   parser has finished parsing the top level type, and only reset the
   type parameter context while we're at the top level.
4. Add test cases for a bunch of partially constructed symbols.

(While I was here, I noticed that a GetMembers() call could be simplified to
GetTypeMembers() and a type check/cast elimiated, so I did so.  The diff
looks better with whitespace ignored because of that).