This removes the majority of references to the deprecated Desktop
assemblies.  The only remaining ones are in the VSIX projects.  They
must remain until we actually delete the assemblies.

VB EnC: when lowering conditional branches produce a pattern that allows the debugger to perform function remapping

sequence of operations to the PDB writing library
Fixes #926
Closes #2342

Created a cross platform solution

This solution file contains the set of projects which we support
building cross platform.

Note: ideally this would be the same as Toolset.sln.  However there are
certain dependencies which we simply can’t use cross platform at the
moment (MSBuild in particular).  Eventually these two solutions should
converge if possible.

Fix stack overflows when running in Mono

Several of our test suites have cycles between static field
initializers and the containing types constructor.  The mono runtime
ends up stack overflowing on these cycles and prevents us from running
suites.  Changing the suites to remove this cycle via lazy field
initialization.

In some scenarios, an attribute application can be “bad” due to an error type in referenced compilation, but without any diagnostics reported for the referencing compilation.

Upgrades System.Collections.Immutable to 1.1.34-rc and
System.Reflection.Metadata to 1.0.19-rc.

Fixes #1942.

Propagate Debugger… attributes from async/iterator method to the corresponding state machine MoveNext method.

This addresses issue #807.

Fix a crash during IL generation for an Expression Tree involving an extension Add method used within a collection initializer.

This change addresses issue #1190. Even though Expression Trees API doesn’t support extension methods as Add methods in a collection initializer, we still compile the code successfully and let it fail at run-time. The motivation is backward compatibility.

For a newly added type, there is no match in the previous generation but we were still looking for its match in some cases like array, type arguments and pointer. This fixes issue #1533.

Add AssemblyRefs and TypeRefs to assemblies and types referenced by imports, even if they are not referenced otherwise

Changed generation of PrivateImplementationDetails to append module name only when dealing with netmudules.

The goal of module name apending is to avoid clashes when combining multiple netmodules into multifile assembly. When building a regular assembly, appending module name is not serving any purpose and just causes unnecessary metadata differences.

Also in this change - when we do apend the module name, replace '.' with '_' when that happens. For example when we build a netmodule and its name is Foo.Bar.dll
More complicated name mangling schemes were discussed, but at this point we will do a simple '.' --> '_' as the least destabilizing change which is still sufficient in the most common case of having dots in the module name.

Fixes #1430

- Remove spurious calls to SymReader.Dispose (per #1539, we'll just let the GC do its job)
- Collect VB tests into subfolder (similar to C#)

We used to have multiple mechanisms - constructing a co-class directly
from a registered GUID, wrapping the co-class in SymReader (for some extra
constant mocking), wrapping the co-class in a TempPdbReader (IDisposable),
and wrapping a TempPdbReader in a DebugInfoProvider (trivial wrapper).

This change eliminates TempPdbReader and DebugInfoProvider and gives
SymReader exclusive access to the raw co-class.

This will be useful when we switch to ISymUnmanagedReader3, which the
co-class does not presently implement (but which SymReader can).

Now that the comment formatting issue is fixed in the Formatter type, we can run the formatter on the remainder of the VB code base.

closes #1424