Add an extension point to allow services to light up on string-literals (classification only for now)

From the #23984 PR:

The first subsystem is called the VirtualCharService deals with the following issue. To the final .net regex, the following snippets of code appear completely identical to it:

"\\1"       // In a normal string, we have to escape the \
@"\1"       // But not in a verbatim string
"\\\u0031"  // The '1' could be escaped
"\\u005c1"  // Even the backslash *itself* may be escaped

These are all ways of writing the \1 regex.

In other words, C# allows a wide variety of input strings to all compile down to the same final 'value' (or 'ValueText') that the regex engine will finally see. This is a major issue as it means that any data reported by the regex engine must be accurate with respect to the text as the user wrote it. For example, in all of the equivalent cases above, there is the same error "Reference to undefined group number 1". However, for each form the user wrote, it's necessary to understand what the right value is to highlight as the problem. i.e.

https://user-images.githubusercontent.com/4564579/34459671-5bb785b2-edab-11e7-8413-79c331ef373f.png

and

https://user-images.githubusercontent.com/4564579/34459672-6deb88dc-edab-11e7-8236-7ba7cd331247.png

So, the purpose of the VirtualCharService is to translate all of the above pieces of user code to the same final set of characters the regex engine will see (specifically \ and 1) while also maintaining the knowledge of where those characters came from (for example, that 1 came from \u0031 in the last example). In essence, the VirtualCharService is able to produce the ValueText for any string literal, while having a mapping back from each character in the ValueText back to the original source span of the document that formed this.

With the VirtualCharService user code can be translated into a common format that then can be processed uniformly. This means that the part of the system that actually tries to understand the regex does not need to know about the differences between @"" and "" strings, or the differences between C# and VB. It also means that it can be used by any roslyn language (for example, F#) if that is so desired.

Add check for symbol.IsOverride when renaming a symbol

Delete Contract.* methods that are equivalent to Debug.Assert

Expose way to add custom JsonConverters to support external Roslyn OOP services

These can be very confusing, since other methods on Contract actually
throw exceptions. Since we have endless amounts of code directly calling
Debug.Assert, let's just use it everywhere.

Prioritize NavigateTo searches to care most about the current actives docs/project.

README update

Update Build.proj

* Precedence: explicit selection -> provider preference -> last selection -> first/default value.
* Remove logic to bail out on retrigger

Merge dev15.9.x to master

Make PinnedSolutionInfo type sealed

Merge dev15.9.x to master

Merge dev15.8.x to dev15.9.x

Merge dev15.9.x to master

* Add AB tests for code cleanup
revert the text change of Code_cleanup_is_not_configured, as the new text is not translated
Only SettingIsAlreadyUpdatedByExperiment to ture for Group B; always show gold bar if user setting is changed in AB test

Add a namespace-glyph when spell-checking+add-using.  

Typos in "SHA1" spelling

Use explicit date format provider to avoid tests breaking in different locales

Make sure TLS 1.2 is used to fetch from https://dot.net

Make sure TLS 1.2 is used to fetch from https://dot.net

Fix rename with F# projects referencing C# projects

* remove random last token from engine.

* put the support back to what it used to, but in different way.

* PR feedback

* added comments on the typing rule.

* PR feedbacks

https://github.com/dotnet/announcements/issues/77 for the change and
where I am stealing this change from.

Disable icacls use on Helix

Remove PR template

We are no longer in ask mode hence no reason to have this around.