Choosing dev15.5.x as the base instead.

Choosing dev15.5.x as the base instead.

This also lets the colors change appropriately when the theme is
changed.

* Fix `is` and pattern-matching behavior in presence of implicit UD conversion
and also an explicit reference conversion. User-defined conversions should
never be considered for `is` and pattern-matching.
Fixes #24522

* Check fully-qualified names for SuppressIldasmAttribute and ReferenceAssemblyAttribute
* Use correct reference location, or fail decompilation if it's not available

Fix and re-enable test that would catch this error

CheckPresenceOfTypeIdentifierAttribute - avoid requesting syntax trees if we know that we already decoded all well-known attributes. (#24576)

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=537870.

Previously we were using xunit.console for desktop tests and dotnet-xunit for our
CoreClr tests. This change unifies us on top of xunit.console (now that it has a
netcoreapp2.0 version available).

also added RoslynActivityLogger that can be enabled through project-system-tool

This was some legacy code that tried to verify that the references
we have from the project system match up to what DTE and other sources
say. This was debug-only, and the actual asserts were commented out.
This is deadweight at this point, so delete it.

It turns out we had some tests, but the tests were disabled. This was
because the tests weren't working properly anyways: they were calling
into UpdateProjectBinPath which only updated some (but not all) of
the project state. That was an internal helper method that shouldn't
be used by tests. Updating the tests to use
SetBinOutputPathAndRelatedData works better.

This field is documented as being written and read from any thread,
but in practice all uses are guarded by an AssertIsForeground(). Thus
we can get rid of the helper methods that are trying to "help" by
locking before accessing the fields, making it really hard to track all
the real uses of it.

* put listener change back in (https://github.com/dotnet/roslyn/pull/24120)

* leave old types in legacy folder until partner teams move to new interface

* added legacy waiter to support partner teams

* fix a case where persistent storage registration fails and some clean up code around it.

* added readonly

* address PR feedback

* removed comments no longer relevant

* renamed lock name