Do not report MA0214 on functions returning only completed tasks

[meziantou]

Fixes #1371

What

MA0214 (Use 'await' instead of returning the task) no longer reports a function when every return produces an already-completed task:

Task FooAsync() => Task.CompletedTask;          // no longer reported
Task<bool> BarAsync() => Task.FromResult(false); // no longer reported

Completed tasks recognized: Task.CompletedTask / ValueTask.CompletedTask, Task/ValueTask FromResult, FromException, FromCanceled, new ValueTask(), and new ValueTask<T>(value).

Functions that mix a completed task with a real task are still reported on every return:

Task<bool> BarAsync(bool condition) => condition ? OtherAsync() : Task.FromResult(false); // still reported

Why

These are "bridging methods", e.g. implementing an asynchronous interface synchronously. Awaiting the task there brings no accuracy to the stack trace and only adds a state machine, so the rule was making the code worse.

Notes for the reviewer

• The detection of completed tasks already existed inside AwaitTaskBeforeDisposingResourcesAnalyzer (MA0100). It moved to AwaitableTypes.IsCompletedTask and both rules now share it; MA0100 behavior is unchanged (its four now-unused symbol properties were removed).
• MA0215 needs no change: for async Task Foo() => await Task.CompletedTask; it already suggests Task Foo() => Task.CompletedTask;, which is the shape MA0214 now leaves alone.
• 8 tests added, including the mixed case (with the fixer output) and a same-named CompletedTask property declared on another type (still reported).
• dotnet run --project src/DocumentationGenerator produces no further markdown change. Full test suite passes on roslyn5.9 (3862 tests) and roslyn4.8 (3750 tests).