Changes our core build scripts to download the CLI / SDK as needed.

Now that we are not tied to roslyn-internal anymore it's possible to
simplify this action a bit:

- Remove the separate script as it's not necessary and can be easily
inlined
- Remove the copy to the root of the source tree. Not necessary anymore
as all our scripts use the returned variable.

Extract code into helper method.

* Add and implement IScriptEnvironmentService

* Remove unnecessary export

Better error reporting for VsLanguageDebugInfo

Fixed IDE0009 suggestion in nameof expression used to initialize static member

Enable publishing of features/readonly-ref artefacts.

Formation completion items after commit if they ask for that.

Properly collapse the right range when encountering nested #if regions.

Be resilient to getting busy messages back from sqlite while retrieving data.

Fix a crash that  could occur in override completion

Allow digit separator after base specifier
Merging on behalf of @alrz. Thanks for the contribution!

Fix documentation comment watson

QuickInfo could crash when showing a documentation comment whose exceptions section included a cref to a constructor.
Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=403665&_a=edit

Merge dev15.4.x to master

Force sign-extending cast when add/sub pointers and int32 in checked context

Simplification should remove parentheses around the expression of an expression-bodied member