Fix issue when pressing ENTER in XML doc comments with a selection

In order to insert XML doc comment exterior trivia (e.g. /// or '''), we grab the caret position prior to the new line being inserted into the editor and then perform a bit of analysis afterward using that position. However, if there's a selection in the editor and the caret is at the end of that selection, the position will be in an unexpected place in the syntax tree, causing that bit of analysis to fail. So, in this case (and all cases), we use the start of the selection as the position to perform analysis on since that position will be consistent after the new line is inserted.

made InternalsVisibleToMoq item hidden in solution explorer

fixed race in task providers

Show property accessor accessibility in CSharp QuickInfo

Add a server mutex to VBCSCompiler

Remove dogfood configurations

Fix mixed encoding

Issue #2790 describes multiple servers running with the same pipename,
which should never happen. It's probable that this was caused by the
File.Exists line in #3177. This change adopts a different strategy
than attempting to check for the existence of the pipe -- instead we
use both client and server mutexes. The client mutex prevents multiple
clients from trying to start multiple servers simultaneously, while the
server mutex acts as a signal to the client that a server is already
running and a signal to all servers unable to grab the mutex that they
should shut down.

As always, any failed server usage simply results in the use of the
in-proc compiler.

Fixes #2790,#3177

Stop setting InternalFeatureOnOffOptions

Change Windows command line argument parsing

Allow #r package reference without explicit version

Minor code cleanup

Add null checks for #load'ed trees...

Reduce VBCSCompiler polling frequency to 1Hz

Add a separate package for the Linux coreclr runtime

Add regression tests for interactive using bugs

...to work around machines that already have the old option set.

Make InteractiveWindow use HACK_ThemeColorFixer

Fix function pointer representation

We were doing so to disable snippets in the interactive window, but it
turns out that it's per-user, rather than per-workspace.

Fixes #5393

Self assignment in expression trees should error

Add support for un-suppressing diagnostics from error list

https://github.com/dotnet/roslyn/issues/4811
https://github.com/dotnet/roslyn/issues/5423
https://github.com/dotnet/roslyn/issues/5450

All assignments in an expression tree should be an error.  Self
assignment was being flagged as only a warning which lead to later
errors in code generation.  Changed the behavior to warn and error for
self assignment (matches native compiler behavior).

close #3826

A process in Windows is provided arguments as a flat string yet main methods in applications receive arguments as an arary of string values.  The transition between the flat string and array is done automatically by the runtime in two phases:

- Break into arguments based on spaces and grouping quotes.
- Remove extraneous quotes and backslashes.

Doing both operations at the same time serves to remove separators that are significant to the compilers.  For example:

1. /r:"a.dll,b.dll"
2. /r:a.dll,b.dll

In the main method both of these argument strings will show up as the latter value (the quotes are considered extraneous by the runtime).  Yet these represent very different values to the compiler:

1. Reference a single file named "a.dll,b.dll"
2. Reference two files named a.dll and b.dll respectively

This change restores the native compiler behavior by manually parsing out the flat argument string.  All of the parsing methods were taken directly from the old compiler.

There are a few small changes of note here:

1. VB now considers semicolon to be a list separator as it does comma (unifies behavior between languages and other tools).
2. VB and C# now have unified behavior for collapsing backslash characters.
3. Fixed several unit tests which were validating incorrect behavior.

closes #4021