Removing diagnostic IDs from previous versions of the VB compiler that are no longer generated by Roslyn.

Internal APIs are aligned with these public APIs.
VB Symbol Display for ByRef returns is adjusted according to the latest design decision.

Provide an API to get unreferenced assembly identities obects from the diagnostics reported about them.

The previous code mistakenly put the TupleElementNamesAttribute on the
type ref for interface implementations and type parameter generic
constraints. This change moves the attribute to the actual interface
implementation and constraint handles, where it belongs.

Fixes #14844

Use tuple syntax in the compiler

Both compilers are able to represent ByRef returns without this special symbol.

This change fixes the following issues in C# compiler: #8948 and #14453.

Fixes #14895

The definite assignment errors were fixed by changing the design of local functions
to allow forward calls to local functions in scope.

Fixes #13029

Also add regression test for #14888. Closes #14888.

This reverts commit 7dd78ae8, reversing
changes made to 449f5586.

Portable unit tests are difficult because as portable libraries they can't truly be built in a runnable fashion.  Instead they need a deployment project to provide a runtime on which to delpoy the output of the build.  To support running our portable tests on both desktop and CoreClr the build is structured as follows:

- Portable tests build w/o deploy to a Dll directory
- Desktop delpoyment references all portable tests and outputs to UnitTests\Desktop\Portable
- CoreClr deployment references all portable tests and outputs to CoreClrTests

This breaks F5 for the moment but gives us the flexibility to restore later.

Fixes #14267.