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.

Fixes #6257.

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.

Hopefully, this will fix internal bug 157535.

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.

Profiling revealed that creation of RuntimeAssemblyManager in the new AppDomain was a significant source of performance issues.  Creating an AppDomain on every use forced us to go through assembly load, JIT, etc ... in the AppDomain on every test.  Pooling AppDomains when there are no load conflicts significantly reduces this problem.

This also revealed a small bug in how we looked up metadata signatures. All loaded modules were being searched instead of the ones actually specified in HostedRuntimeEnvironment.  That is also fixed with this change.

Optimize the data passing across AppDomains to not pass ModuleData for
already loaded Assembly values.  ModuleData is relatively expensive to
marshal because it has the full byte image of the Assembly.  No need to
marshal this data for pre-loaded assemblies like mscorlib, System.Core,
etc ...  It is sufficient to verify the ModuleDataId is already present.

This cleanly separates out all of the data produced by emit into another
type.  This type now has a single access point that throws when
improperly used.

This cleans up the emit order for references to ensure Compilation
values are always emitted as Compilation.  Indirect values that refer to
the Compilation simply use the data emitted by the Compilation.

The code now more accurately tracks that the ModuleDataIds do not
conflict.  It now validates that both simple names and MVIDs match while
previously it just matched simple names.

This code also fixes a bug, caught by the earlier check, where
Compilations that appeared as references multiple times would emit
multiple modules.  Now it emits a single module.