Entered tracking bugs instead as customary when merging into Master branch.

Previously the syntax generators wrote all of the output into a single file.  That file was quite large and was difficult to use in tools like source.roslyn.io.  Splitting the file into three in order to make that tooling better.

* 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.

Similar to PR #12611. This PR implements metadata roundtripping for
tuples used in generic constraints. Completes the generic parameter
constraint work listed in #12347.

* Add support for a new ProjectShimFactory to be consumed by CPS.

1. IProjectShim and IProjectShimFactory are the primary interfaces being introduced that CPS will use to forward project system changes to workspace hosts.
2. Refactored the UpdateOptions code in AbstractRoslynProject and sub-types so that it can be shared with the new project shim.

TODO: Add IVT to CPS binaries to enable consuming these APIs (need to follow up with DavKean)

* Add support for a new ProjectShimFactory to be consumed by CPS

1. IProjectShim and IProjectShimFactory are the primary interfaces being introduced that CPS will use to forward project system changes to workspace hosts.

2. Refactored the UpdateOptions code in AbstractRoslynProject and sub-types so that it can be shared with the new project shim.

3. Add IVT to CPS projects to enable consuming these APIs.

* Fix tests

* Add tests for new CPS project shim.

* Fix the tests and update the Shim factory API to take CommandLineArguments.

* Fix VSI test - VB compilation options must also contain latest parse options

* Some more code refactoring to ensure that for the CPS shim we avoid getting properties through the hierarchy while creating the shim and adding new source files.

* Some more code refactoring to address review feedback:

1. Rename IProjectShim and IProjectShimFactory to IProjectContext and IProjectContextFactory respectively.
2. Add a new sub-type of AbstractProject, namely "AbstractLegacyProject", for legacy C# and VB project system shims. All the Hierarchy accesses for computing properties have been moved to this sub-type.
3. Move legacy project system specific interface implementations (IVsHierarchyEvents, IIntellisenseBuildTarget and ICompilerOptionsHostObject) to AbstractLegacyProject.
4. Merge AbstractRoslynProject into AbstractProject - the only addition in AbstractRoslynProject seem to be support for VsENCRebuildableProjectImpl, which was anyways an optional field.

* Change the name of the flag taken by AddSourceFile to "isInCurrentContext"

* Address review feedback

* Fix tests broken by last commit

* Further cleanup of options setting infrastructure

* Minor refactoring of the Options setting APIs

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.

Currently, if you implement an interface with a tuple type as a
substituted type argument that will be accurately represented in source,
but the names will be lost when emitted to metadata.

This PR implements roundtripping for tuples as generic type arguments to
interface implementations in metadata. This is done by adding support
for emitting attributes on interface implementations to Emit. We now
emit the same attribute on interface implementations that we would
on other areas where types may contain nested tuples. Completes the
interface implementation work referenced in #12347.