Creates a new package, Microsoft.NETCore.Csc, which contains a Csc
binary compiled against the netcoreapp1.1 shared framework. This can be
installed as a NuGet package and built using the dotnet SDK, as long as
the netcoreapp1.1 framework is installed.

This change also downgrades a few packages, like
System.Diagnostics.FileVersionInfo, that were higher than the
version available in netcoreapp1.1.

# Conflicts:
#	src/Compilers/VisualBasic/VbcCore/project.json
#	src/Tools/Source/CompilerGeneratorTools/Source/BoundTreeGenerator/project.json
#	src/Tools/Source/CompilerGeneratorTools/Source/CSharpSyntaxGenerator/project.json

* Writing attribute to metadata

* Reading attribute in PE symbols

* Added more tests for lambdas, delegates, and other types

* Use ReadOnlyAttribute instead

* Fix build break

* Fix Failed Tests

* Enable back disabled tests

* Fix more tests

* Ban usage of ReadOnlyAttribute in source

* Rename System.Runtime.InteropServices.ReadOnlyAttribute to System.Runtime.CompilerServices.ReadOnlyAttribute

* Clean up

* Address CR comments

* More PR feedback

Implements the initial set of span-safety checks.

This will be allowed only for span-like containers eventually, when we can declare those.
For now allow in any struct.

These are mostly the checks dealing with stack-only nature of the Span. With minor difference they follow the same logic as the existing checks for types such as TypedReference.

==== Not in this change:
Defining and detecting span-like types is NYI. For now we just treat any type named "System.Span" and "System.ReadonlySpan" as span-like. This will change.

Some of the checks result in somewhat generaic messages and happen at emit phase. That was ok when the failures were supposed to be rare.
Error clarity is not the goal of this change, but we will examone what errors should say and whether they should be moved to an earlier phase.

Merge from master to features/readonly-ref

* Make sure errors are consistent when providing null vs empty key files during signing

* Addressed PR Comments

* Treat empty CryptoKeyFile as null

* More PR Comments

Add integration tests for F1 help

Add VSI tests for Navigation Bars

* Add integration tests for keyword highlighting

* minor cleanup

* Use new APIs

* Verify highlight spans

* Delete extra file

Add a test for VB line commit

Merge dev15.2.x to master

Add option to run just netcore integration tests

Fix null ref in initialize-parameter.

Revert "Disable BasicIntellisenes.TypeAVariableDeclaration until #18397 is fixed"

Disable BasicIntellisenes.TypeAVariableDeclaration until #18397 is fixed

No scrollbar in Code Style preview pane

Make the "Description" column in Code Styles Options a "dataitem" control

Improvements to IOperation test dumper