* Implement source link feature

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

* Reduce code duplication across VB and C#

supported language version when used in APIs.
Also update the C# 7 feature status doc to bring it up-to-date.

We also change the default language version for VB from 14 to 15, and
put the binary literals and digit separators under language version 15.

This PR takes the approach of immediately transforming a "Latest"
language version to the actual latest language version. Thus, the extra enum value
should never be persisted as part of an options object, but only transiently
appear as an argument to an API call setting or resetting the language version.

Also address review comments with more tests

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

This reverts commit d4d502e4.

Apply feedback from Pilchie
Apply feedback from CyrusNajmabadi
Missed feedback from CyrusNajmabadi
Apply more feedback from CyrusNajmabadi and AlekseyTs
Apply feedback from AlekseyTs
Apply feedback from jcouv

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

This reverts commit 7debfc47.

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.