Global notification when adding/removing projects

Fix calculation for slot containing offset

Suggest adding Microsoft.VisualBasic for ERR_MissingRuntimeHelper

make solution crawler to cancel all running tasks on global operation

Fixes #443.

Remove some dead code

Implemented incremental computation of ChangedText.Lines

This addresses issue #608 from @mkosieradzki.
Note that the Visual Studio IDE scenario will not be impacted by this change because, when hosted in Visual Studio, we use editor TextBuffer snapshots as the backing store and they already do their own line tracking.

Disable the severity menu items when a selected diagnostic is not configurable

previously on global operation, soluton crawler will pause itself but it won't cancel already running tasks.

this change make solution crawler to even cancel running tasks and go to pause mode sooner on global operation.

cancelled tasks will re-enqueued to work queue and will run next time.

VB does not support virtual events.

In the dev12 PDB format, usings and imports were encoded as strings in
PDB files.  The strings were structured, but required a certain amount of
guesswork (for example, you might not know whether you were importing a
type or a namespace until after binding).  In the new PDB format, usings
and imports are represented as structured blobs and tokens are used
wherever possible.

This change introduces two new class hierarchies, ImportRecord and
ExternAliasRecord, that the expression compiler can pass around instead
of raw strings.  Unfortunately, there is still switching logic in two
places - both when these types are instantiated and when they are
consumed.  The instantiation switch is very simple - we just check for
the appropriate version of ISymUnmanagedReaderX.  The consumption logic
is more complex, because the new and old representations differ
substantially.

TODO: Presently, there is very little code for consuming the new PDB
format, since the APIs are not yet available.

In string interpolations, parentheses can't be removed if they surround
some expression that includes a : or :: token. Othewise, a parsing
ambuity will be introduced due to the : that delimits the format clause
(i.e. $"{number:x4}").

Fixes Issue #724.

Clean up projects building against FX 2.0

My prior change #673 made to address issues with analyzer exception diagnostics introduced a few leaks in product and test code. The primary reason was that I attempted to use static events to track state as the existing AnalyzerDriverHelper type which did the core analyzer execution was a static type with all static methods. Additionally, I added a static event to the new host diagnostic update source added for reporting analyzer specific diagnostics. These were holding onto projects/workspaces in test runs causing leaks.

I have reverted the approach of static events and instead refactored the code in AnalyzerDriver project to simplify the whole design:
 1. Renamed AnalyzerDriverHelper to AnalyzerExecutor and made it a non-static type, which has instance fields for all the configuration parameters for analyzer callbacks.
 2. Move all the core analyzer callbacks (actions/Initialize method/supported diagnostics) into AnalyzerExecutor. Command line compiler just creates a single instance of the executor, while IDE driver creates instances per analyzer. 
 3. Both these changes simplified the API a lot, and I just had to add an additional field "Action addExceptionDiagnostic" to AnalyzerExecutor to configure how to handle exception diagnostics.
 4. MEF import AbstractHostDiagnosticUpdateSource in DiagnosticAnalyzerService and thread it down to IDE analyzer driver.
 5. Changes 3 and 4 above meant that in the IDE driver, delegate "addExceptionDiagnostic" just asks the HostDiagnosticUpdateSource to report the exception diagnostic produced by the AnalyzerExecutor.

This change also re-enables the tests skipped by #761 and fixes #759.

Move Pdb2Xml from closed to open

Harden EditAndContinueMethodDebugInformation deserialization

BaseDiagnosticIncrementalAnalyzer doc comments

Added doc comments to the BaseDiagnosticIncrementalAnalyzer class sufficient to guide a new implementation of an incremental analyzer engine.

(Also, this makes for a relatively safe set of changes for my first pull request merge.)