* Implement source link feature

* Fix CommonCompiler to open source link stream, add E2E tests

* Reduce code duplication across VB and C#

Revert "Revert "Revert "Revert "*DON'T MERGE INTO MASTER* Hide IOperation API behind a feature flag""""

This reverts commit d4d502e4.

[Based on features/async-return with tests ported from that branch.]

For the most part, this change implements the same encoding
for tuples that we use for dynamic. This means that interface
implementations and generic constraints are not covered, but
base types and type parameters are.

This change also doesn't have full tests for "long" tuples (nested tuple
transformations) or tuple types that can only be constructed in
metadata.

- Substitute underlying type with corresponding tuple type at the point of type reference.
- Adjusted implementation of symbol display to account for presence of names in the extension tuples.
- Implemented proper retargeting for tuple types (multi-targeting support).

- Include memebers of underlying type into a set of members returned by
tuple type symbol. Properties and events are not supported at the moment.
- Track declaration locations for tuple related symbols.
- Ensure Rest extensions are tuples.

Ensure that we generated a CompilationCompletedEvent for compilations with no source files (example an empty VSIX projects).

Ensure that we generated a CompilationCompletedEvent for compilations with no source files (example an empty VSIX projects).

This reverts commit a96d7825.

This reverts commit 92ff3f8d.

Fixes #9759

Fixes #9759

This reverts commit b03e8478.

https://github.com/dotnet/roslyn/commit/b03e84783806dc48f240e56188e6473323dc3ba1

The flag name is "IOperation". Register opeartion actions and calling
SemanticModel.GetOperation while the feature is disabled will throw an
InvalidOperationException. Also log a non-fatal-watson when
SemanticModel.GetOperation crashes.

Fixes #7477

and use that to diagnose cases where the compiler produces
compilation events later than it should.

Scripting scenarios set a special flag in the ReferenceManager to enable
additional tie-breaking rules in the presence of conflicting references.
Enable that same flag for the EE, which also routinely encounters
conflicting references.

Fixes internal bug 161294.

Fixes #8178