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

This change splits the Workspaces layer into two parts, mirroring the Portable/Desktop split that the compilers did. The core parts of the Workspaces layer (managing documents and projects, formatting, some refactoring, code fixes) is kept in the portable subset, with a few non-portable pieces remaining, notably MSBuild support.

This change has a major impact on how MEF now works in Roslyn. Traditional MEF (“MEFv1”) is not portable, and so we must move the Workspaces layer over to using the Microsoft.Composition NuGet package (“MEFv2”) which is. The APIs are distinct in that each has its own namespace, but the concepts are more or less identical. It requires some care though: the workspaces layer is simple in that it only references MEFv2, but higher layers have to reference both versions to use metadata attributes. Exports using metadata attributes from the editor (say, ContentTypeAttribute) must use MEFv1 attributes to export, whereas exports for the workspaces layer must use MEFv2 attributes. The rule is subtle and yet simple, and so a diagnostic is provided which catches any offenses to prevent confusion.

This also has some impact in how we create MEF hosts: if you wish to host just the base workspaces layer, we can use MEFv2 to compose everything. The HostServices implementation (MefV1HostServices) that consumes a MEFv1 ExportProvider. If you’re in Visual Studio, you can use this implementation to get the full set of host services provided in Visual Studio.

Otherwise, most of the changes in here are minor: we react to some APIs that have been moved/renamed in the portable subset we are targeting, and also align our various exception helper utilities with the compiler’s precedent. (changeset 1349276)

Devdiv:756226 - Quick info for Await Keyword.
Devdiv:756337 - (awaitable) not displayed for types that implement the awaiter pattern.

     Roslyn behavior now models after Dev12, which
     1. shows the type of the await expression in quick info for the Await keyword
     2. appends an <Awaitable> prefix if the expression is further awaitable. i.e., it has a type of Task or Task<T> or any type that has a valid GetAwaiter method.

     Since the changes are shared code, VB also inherits this behavior and shows semantic type information. But, in Dev12, VB shows syntactic help for Await keyword in quick info (Await <expression>).  Syntactic quickinfo for keywords are Not yet implemented in Roslyn (user story: 522342). I have added tests to document the current behavior and leave the behavior as such until we implement 522342, when we can decide what the quick info text should be like for VB Await keyword (syntactic or semantic or both)... (changeset 1278712)

FeaturePacks have changed to become HostServices.
IWorkspaceServiceProvider has become HostWorkspaceServices
ILanguageServiceProvider has become HostLanguageServices

HostWorkspaceServices can be accessed directly from Workspace.Services.
HostLanguageServices can be accessed directly from Project.LanguageServices.

Some of the existing workspace and language services have been made public. (changeset 1226729)

Public API Change: Rename IFieldSymbol.AssociatedPropertyOrEvent , IMethodSymbol.AssociatedPropertyOrEvent and INamedTypeSymbol. AssociatedEvent APIs to AssociatedSymbol to accommodate for future language changes. (changeset 1210026)