Merge dev15.7.x to dev15.8-preview4

Reduce IO costs in AnalyzerFileWatcherService

Defer reading of the metadata timestamp until later

Enable refactorings which were unintentionally removed by 'none' severity

Change 'None' in code style options to 'Refactoring Only'

this will help us track some ratio such as completion commit/dismiss

Only call EnsureSubcription for text loaders when we are actually reading

Consolidate diagnostics IDs for "use builtin/framework types".

The following refactorings failed to consider cases where the corresponding diagnostic
was disabled via the 'none' severity in .editorconfig. For each of these cases, no
refactoring was provided if the style is configured as the preference for generated
but the diagnostic is disabled.

* Use Implicit Type
* Use Explicit Type
* Use Expression Bodied Member
* Use Block Bodied Member

When we were creating new metadata reference snapshots, we were reading
the timestamp far earlier than needed; all that matters is we actually
read it prior to reading it.

This change is a fairly paranoid change as I try to preserve existing
semantics (and perhaps bugs too). This area is quite sensitive.

This service tries to watch analyzer files to see if they've changed,
and if they have inform the user that they'll have to restart Visual
Studio. It did this via two ways:

1. When a file was first added, it's modification time was stored in
   a dictionary, which was checked in any subsequent entry.
2. Via file watchers.

The first approach was broken in an interesting way: each time a
reference was added, we'd add the modification time to the map (doing
the IO to get the time). This overwrote the previous value, even if
the value had changed in the middle. Then, we'd do the IO a second
time, checking against the value we had just stored. As a result, the
window where we could detect a change in the first approach was very
tiny.

I attempt to rectify what seems to be the intent here and also speed it
up. First off, we won't overwrite previous values so the first approach
has a better chance of actually working. Also, we'll read the data
once instead of twice. Further more, once the file watcher (second
approach) is active, we'll just stop reading timesetamps entirely,
because by then there's no reason to use the first approach at all.

Note this approach still has a flaw: if the file is modified between
when we do any timestamp checking, and the file watcher is active,
we'll completely miss the change and report nothing, at least until
somebody tries adding the analyzer again. This isn't new; it seems
this was already broken anyways. We could just always force the file
watcher immediately, but that might be tied up if somebody else is using
the service so we're still assuming IO is cheaper in that case.

servicing fix for CLR_EXCEPTION_System.NullReferenceException_80004003_Microsoft.VisualStudio.InteractiveWindow.dll into dev15.7

* Use System.Runtime.Loader light-up package

Expose IOperation.SemanticModel

bug fix: don't optimize interpolations when in an expression lambda

Remove testoperationvisitor workaround now that bug is fixed.

Skip 3 failing NetCoreTests until fixed

Unskip test for fixed IOperation issue

Move to new MSBuild drop

Original issue: https://github.com/dotnet/roslyn/issues/23821

Correct doc about Equivalence Key

Test alias to forwarded type

We were calling .EnsureSubscription() right away when we add the
documents to the workspace, which was never the intent of this API;
this can (and should) be deferred until when we read the file.

Stop calling IVsHerarchy.ParseCanonicalName if we don't need it

* Only import package versions in projects that restore toolset packages

* Add nuget.org feed