Tuples: adding feature flag

(Literal, Lambda, InterpolatedString)
They do not have "Expression" at the end.

Tuples: lookup well-known ValueTuple members relative to given underlying type (not well-known type)

Made tuple ctors WellKnown members
Made ctors optional in the bound tuple creation nodes
ObsoleteAttributeData for tuple wrappers returns null (at least for now)

Rebased onto future branch
Fixed Tuple field signatures
Fixed tests affected by tuple parsing

Most interesting parts here are:
* the syntax node shape and naming - these are public APIs so we should get the naming right.
NOTE: I expect some churn here so for now I have disabled the public API diagnostics.

* parsing something that starts with "("  is tricky since that can be a start of many constructs - lambdas, casts, parenthesized expressions...
I have updated the lookahead logic that tries to figure what we are dealing with, but it may need some tuning to be more robust.
In particular there could be some error recovery work to see that handling of broken code is sensible.

==== Current assumptions:
1) TupleTypes and TupleExpressions must have 2 or more elements/arguments. However we may have to parse 0 and 1 sized expressions for error recovery reasons. Those are syntax errors.

2) TupleType looks like

(int x, int y)  x = ...

I.E.

  ( type [name], type [name])

We may enforce that all or no elements have names. Not sure yet if that needs to be in parser or binder.

3) TupleExpression looks like

var something = (x: foo(), y: bar);

I.E.

  ( [name:] expr, [name:] expr)

just like with the types we may want to enforce that all or none of the arguments have names. Not sure if that is a syntax or binding error.
It is not a part of this change.

This reverts commit 15488921.

Only exception is params parameters, tracked by #10708.

Fixes #10389

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

This reverts commit 7debfc47.

This reverts commit a03d8b8e.

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