Most callers are probably interested in
`Compilation.ScriptCompilationInfo.PreviousScriptCompilation` as well.
Behavioral changes expected in QuickInfo (fix crash) and SymbolFinder
(offer additional CodeFixes).

Fixes #6616 and #6657.

 1. Replace the commas to avoid disrupting csv files.
 2. Prepend the type names, since they don't seem to be included by
 default.

Explicitly states that the change is a workaround against a debugger bug.
Moves the state cleanup to a top level finally clause.

Any exceptions caused by the EnC project become non fatal.

This reverts commit c25f2528.

Windows erorr reporting shows reports of crashes due to LockRecursionException
due to the following pattern:
* ASP.NET design view forces a lazy loaded tab to be populated during solution load
* That causes us to need to start pushing to workspace hosts in
  VisualStudioProjectTracker.StartPushingToWorkspaceAndNotifyOfOpenDocuments
* Ends up waiting in Workspace.OnDocumentOpened.
* Waiting pumps, and ASP.NET is finished parsing, so it tries to set the compilation options
* Workspace.OnCompilationOptionsChanged throws because the lock is the same one being waited
  on by Workspace.OnDocumentOpened.

Fixes one of the causes of internal bug 149765.

Object Browser was forcing compilations to be realized just to compute the assembly identity for each of a compilation's metadata references. With this change, the Object Browser will use the metadata references from the project and call the same utility function that is used to read assembly identity for analyzer assemblies.

Move the files in the compiler that are shared by source with other projects into the Shared folder.  This combined files which weren't in a shared location and those previously in Helpers.

When `ExitBreakStateOnPE` is not called for each project VS would normally
fail the next instance `EnterBreakStateOnPE` is called.
This change prevents the crash.

Fixes #873

Sometimes there are buffers with our C#/VB content types that are not added to the Running Document Table and therefore never end up in the MiscellaneousFilesWorkspace (or any workspace), such as the Source Control "Annotate" view. This change updates the Syntactic Classifier to handle such cases by parsing its initial snapshot for tagging (changes are not observed, but the known use cases of this fix are uneditable anyway).  There are some exceptions to this behavior (e.g. the Immediate window) which require us to go digging through attached views, but we default to classifying these buffers now.

This change disables AppDomain isolation in the unit tests that don't require it.  This is necessary for our goal of running many of the suites on CoreCLR and helps reduce our exposure to #6358.

The AppDomain creation code incorrectly used AppDomain.CurrentDomain.BaseDirectory as the new base directory.  There is no gurantee this points to the directory containing the unit test binaries and this is actually the case in xunit without AppDomains.  Instead it points to the directory containing xunit.oconsole.x86 and uses AppDomain.ResolveAssembly / LoadFrom to resolve unit test binaries.

Switching over to no AppDomains in xunit was causing the creation of RuntimeAssemblyManager across AppDomains to fail.  The error given was a failure to load Roslyn.Test.Utilities.Desktop in the original AppDomain.  This is odd given that the code is executing inside Roslyn.Test.Utilities.Desktop.  The error was fixed by hooking AppDomain.AssemblyResolve in the original AppDomain and essentially reloading the DLL.  I'm not completely sure why this worked but I suspect it has to do with the original AppDomain having an incorrect base directory.

since this logic is now exposed to outside of engine, engine needs to be guarded from abuser so, logic is added to drop cache if there is too many high priority work. before only engine can decide what is high priority or not.