Breaks the internal GTD infrastructure into seperate symbols for
determining the navigable symbol at the caret position and performing
the navigation.

If we're going to require it, let's at least crash with a useful
exception instead of a null ref.

Revert "Merge pull request #20558 from Pilchie/Fix19968-DuplicateSourceAndAdditionalFiles-DistinctIds"

This reverts commit 6f5b2d47, reversing
changes made to 66362df5.

This is a breaking API change.  Need to decide if we want to take it or
not.

This reverts commit 7f41dad5.

This reverts commit 95a24448.

One path in the VB type inferrer assumed that invocations of Select() would always have an argument list, leading to crashes. We now check that it is not null before accessing it. This is the only unguarded use of this property in the type inferrer.
Fixes  https://devdiv.visualstudio.com/DevDiv/_workitems?id=431509&_a=edit&triage=true

Add option to disable OOP diagnostics during testing.  They don't currently work with the CodeAction testing infrastructure.