Move to a different lowering from for nullable value types to work around a bug in TransformCompoundAssignmentLHS. Addressed PR feedback.

Fixes #36346

Fixes #36018.

Replace `dynamic` wih `object` when substituting constraints.

Fixes #36276.

Remove support for explicit `object` generic type constraints

Introduces optimizations in lowering for an indexer access of the form
receiver[a..b]. With the optimizations, the generation of intermediate
System.Index and System.Range types will be avoided.

* Dont try and check tuple conversions in error cases:
- When checking an identity converison, also check for a bound tuple if the type isn't a tuple as we might have a bound tuple with an error type
- When checking a tuple conversion, don't GenerateConversion in the error case as we don't have a target type to generate against
- Update tests

Update ??= to convert the result type of the expression to the underlying value type when targetting a nullable value type.

* Use ref locals to simplify code flow
* Fix VB code layout

Works around microsoft/vs-streamjsonrpc#272

* Ctor refactoring

* Renames

* Renames

* Cleanup

* ProjectExternalErrorReporter ctor

* Refactoring

* Remove dependency on VS

* Aggregation improvements

* Use ordinal comparer for project names

* Construct ProjectExternalErrorReporter lazily

* Fix off by one

* Fix issues

* Update implicity declared type expression nullability:
- When updating the implicit type in a declaration also update the bound type declaration nullability
- Record nullability for inferred foreach variable
- Add nullable public api tests
- Add nullable tests

* Learn from comparisons to non-constant, non-null values

* Update comment

* Remove unused local function

* Remove another unused local function

* Check for null constant before trying to learn from notNull == maybeNull. Add tests

* More tests, more cleanup

* Add conditional access with || test

* More test updates from feedback

* Cleanup

* Change some locals in tests to parameters

Add initial support for 'notnull' generic type constraint.

See https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-05-15.md
Also fixes #36005.

Don't read the same property twice in pattern matching when accessed through the same underlying object, but references of different types.
Fixes #34933

This change produced a 535MB (7.5%) allocation reduction in the scenario
described by #36114.

Add Default Interface Implementation tests for VB

Related to #35820, #35821, #35823, #35824, #35827, #35834, #35852, #35885, #35911, #35948.