Merge dev15.3.x to master

Merge dev15.6 back into master

LocalFunctionTypeParameterSymbol isn't necessary as long as the functionality around type constraints can be moved into a shared base class between ordinary method symbols and local function symbols.
In fact, it would be very useful to have a shared base symbol aside from MethodSymbol between local functions, lambdas, and other source method symbols. To this end, I've renamed SourceMemberMethodSymbol to SourceOrdinaryMethodSymbol (since it is just used for ordinary member-level methods), SourceMethodSymbol to SourceMemberMethodSymbol (since it is only used as a base class for member-level method-like functions like ordinary methods, constructors, property accessors, et al.), and added a new abstract class SourceMethodSymbol that is the root of both LocalFunctionSymbol and SourceMemberMethodSymbol. This lets us unify all source method-like symbols under a single base class, SourceMethodSymbol.
The first three commits perform the refactoring, while the last commit actually removes LocalFunctionTypeParameterSymbol and moves the necessary functionality into SourceMethodSymbol. I expect more functionality to be moved over in subsequent PRs.

Fixes #17244

Don't generate expression bodied accessors in C# 6.0

Avoid going to the database to compute a value that we already have in memory.

Initial work to provide a code style for ordering modifiers.

Set charset in .editorconfig

Merge master to dev15.6

Fix getting package references service

Override object.ToString in Optional<T>

We're trying to go from an `IVsBrowseObjectContext` to an `IPackageReferencesService`.
We were doing this through `IVsBrowseObjectContext.ConfiguredProject`, but it turns out
that this property always returns null on the particular implementation of
`IVsBrowseObjectContext` we're working with.

Instead, we need to go through the `UnconfiguredProject` and ask it for the "suggested"
`ConfiguredProject`.

I've also switched from `GetAwaiter().GetResult()` to properly async/await code inside
an `ExecuteSynchronously` call, mostly for the purposes of readability.

Move remainder of Roslyn to new SDK

Set insert_final_newline to true for C# and VB source files

Since tests are run in a subfolder of the solution, they can currently see
the changes we make to .editorconfig.

Introduce a more flexible service to allow languages to plug into common comment/uncomment behavior