The semantic model often creates ReducedExtensionMethods for the public API,
but the creation of ReducedExtensionMethod does constraint checking, which
requires a compilation, which isn't always available. This change removes constraint
checking from all creation of ReducedExtensionMethod and adds asserts that we
always have a compilation during constraint checking.

Fixes #37021

Implements most of the design changes specified in
https://github.com/dotnet/csharplang/blob/c229cae634bd59a6a13b9ed464a4cab782a95e5d/proposals/index-range-changes.md

This PR focuses on getting the simple end-to-end scenario working,
not focusing entirely on codegen quality. I expect to follow-up later
with the optimizations mentioned about eliminating use of the
Index/Range helpers entirely if they can be elided.

Report a better error for an attempt to access protected member of object class within an interface.

Before this change, the following confusing error was reported for all error locations in the added test scenario:
CS1540 Cannot access protected member 'object.MemberwiseClone()' via a qualifier of type 'object'; the qualifier must be of type 'I1' (or derived from it)

Captures Invalid Dimensions from an array variable declaration and adds it to IVariableOperation

.Net - .NET

e.g. `ReturnType`, `ElementType`, etc.

Update the stub API to remove Nullability and replace with NullableAnnotation and NullableFlowState, and update the
the rewriter to use that instead of the Nullability enum.

- Rename `TypeSymbolWithAnnotations` to `TypeWithAnnotations`
- Rename its type field from `TypeSymbol` to `Type`
- Fields of Symbols that are of type `TypeWithAnnotations` that are currently named `Type` would be renamed `TypeWithAnnotations`
Fixes #33736

https://github.com/mono/mono/issues/12603

Merge remote-tracking branch 'origin/features/editorconfig-in-compiler' into merges/master-to-features/editorconfig-in-compiler

Standardize the conditional IOperation verification skip method, and fix a bug that caused it to skip in exactly the wrong way.

Don't warn about "!", regardless of context (https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-10-29.md).

Closes #30177.
Closes #30021.

The target framework netcoreapp2.0 includes a new type: `KeyValuePair`.
This conflicts with our `KeyValuePair` utility class and causes a lot of
friction whenever netcoreapp2.0 is added as a target framework for a
project.

Renaming the type to `KeyValuePairUtil` to remove the ambiguity.

This updates our duplicate reference check to compare names in addition
to reference identity. This found ~600 tests which were inadvertently
passing duplicate references to our compilation helpers.

closes #25509

Also add
- CommonConversion.IsImplicit
- Compilation.HasImplicitConversion
Fixes #9461

This change allows the user of a Compilation to configure warning
reporting on a syntax tree level. There is a new public API, 
DiagnosticOptions, on SyntaxTree which allows a user to retrieve
these options in a map from diagnostic ID to severity.

In general, these options take precedence over the global diagnostic
options, but are overridden by pragmas. In C# the global warning
level can also suppress these options.

* Do not report unused using/imports if not processing doc comments.
Also silently process doc comments when parsing them so we can report unused usings.
Fixes #2773

* Inaccessible extension methods should not be considered eligible candidates.
Fixes #25813