* x == default is valid object equality for reference types
* Block null == default

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

- 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

* Move Index from unary operators to a stand alone node

* Remove indexer hack

* Remove prototype comments

* Testing/fixing IDE services

* Added EE tests

* Rename "Index" nodes to "FromEndIndex"

* More comments

* Some minor fixups and test fixes

* Parsing index operator

* Binding index operator

* Parsing range expression

* Binding range expressions

* Range IOperation

* Fix Build Break

* Lowring indexes and ranges

* Fix failing tests

* Extension Indexers hack

* Address PR comments

* More PR Comments

* Clean up

* Prefer by-val methods over in methdos in overload resolution

* More tests

* Moved check to the end of OR chain

* Address PR comments

* Operators overloading

* Address PR Comments

* Clean up

Type inference with nullable reference types

Here is the link to the notes https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-06-27.md.

Support declaring operators ```+ - ! ~ ++ -- true false * / % & | ^ << >> > < >= <=``` in interfaces. (#20401)

(The original incorrect optimization is from PR #9543)
Fixes #13380

* Move Roslyn to newer toolset packages

1. Move to 1.2.1 compiler toolset package.
2. Move to 1.2.0-beta1 analyzer packages.
3. Turn on IOperation feature flag for all projects as the new analyzer packages have IOperation based analyzers.

* Use the Roslyn wide default CA rulesets for newly added samples project.

* Fix couple of test projects to use the Roslyn wide rulesets for code analysis.

* Move to analyzers 1.2.0-beta2 packages (multiple fixes and enhancements to Roslyn diagnostic analyzers)

* Revert accidental file mode changes.

* Move shipped implicit constructors to public API shipped files.

Fixes #11446

Note that the test passes due to smoke and mirrors, not because `operator is` works.

- avoid instantiating nullable type when doing operator overloading on enums
- avoid instantiating generics when checking for impossibly rare case of duplicated interface implementations
- skipping materializing of sequence point array

At the moment we just follow the definite assignment logic. The majority of the work is to make sure proper flow of nullable annotations from target delegate types to lambdas and to make lambda binding caches aware of the annotations.

Add ability to associate annotations with type symbols exposed by other symbols. For example, with property's/method's return type, parameter's type, etc. As part of this change, custom modifiers are now handled as an annotation.

In a switch's default clause that is unexpected, for example, we should
    throw ExceptionUtilities.UnexpectedValue(value);
Long ago this was a standard but we've drifted from it.
Closes #3495

Related to #846.