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

Remove support for explicit `object` generic type constraints

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

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.

Support re-abstraction of interface members in derived interfaces

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* Fix up XLF files

* Clear AssemblyVersion

Implements a design change where overrides are allowed to change the
type of the member as long as there is an implicit nullable reference
conversion from the overriding type to the overridden type according to
the nullable variance rules.

Fixes #23268
Fixes #30958

Related to #32054.

* Permit stackalloc in nested contexts. (#28969)
Fixes #26759

Make sure nullability mismatch in constraints specified in different partial declarations (types/methods) are properly detected and reported. (#35272)

 Make sure nullability mismatch in constraints specified in different partial declarations (types/methods) are properly detected and reported.

Fixes #30229.
Fixes #35179.

Implements the following LDM decision:

For partial types, the invariant matching from type inference and merging. A mismatch
between two non-oblivious candidates produces an error. No warnings are produced.

For partial methods, nullability has to match with exception for oblivious and we produce warnings.
For the result, we use the implementation signature inside the implementation, and the
declaration signature for the callers.

Fixes #29894

Implement `where T: class` and `where T: struct` for overrides and explicit interface implementations.

Closes #34798.
Closes #29847.

Restore some resources incorrectly removed from the .xlf files.

Implement `where T: class` and `where T: struct` for overrides and implementations

Correctly report mismatches in foreach types, and adjust location of the warning to apply to the iteration variable.

Fixes #33359

Fixes #34678

* Check parser behavior for bad expression property declaration

* Make readonly modifiers more strict on accessors to match accessibility behavior

* Fix tests

* Improve implicit copy warning message. Add compound property assignment test.

* Test ErrorCode.ERR_ReadOnlyModMissingAccessor with an indexer

* Simplify error message for 'static readonly' members

* Implement pattern-matching in the nullable walker

Fixes #29909
Fixes #31881
Fixes #30952
Fixes #33499
Fixes #30597
Fixes #32414
Fixes #23944

* Remove infinite recursion by using an empty struct cache.

* Changes per code review comments.

* Remove debugging code accidentally left behind.

* Analysis of patterns-matching in the nullable walker requires valid (>0) slots.

* Skip a flaky test

* Patch after merge.

* Make ctor private to force use of factory methods

* Correct a typo.

* Fixup after merge.

* Use RefKind.In for the ThisParameter of a readonly method

* Remove prototype comments

* Add readonly lambda test

* Fix formatting

* Update compiler test plan

* Tweak wording

* Move OverrideBaseMethod test to CodeGen tests file. Add 'fixed' test.

* Test readonly partial methods

* Add readonly explicit interface implementation test

* Expand on ReadOnlyMembers_BaseMethod code gen tests

* Check symbol APIs for mismatched readonly partial method

* Update speclet

* Respond to feedback

* Refactor metadata round trip test

* Add IDS_FeatureReadOnlyMembers

* Add CheckFeatureAvailability checks for readonly members

* Remove prototype comment

* Ensure LangVersion test is valid with default language version

* Fix DeclarationParsingTests

* Remove unnecessary comment

* Add new readonly members error codes

* Use new diagnostics and update tests

* Improve ValueChecks error messages

* Remove unused error messages

* Fixes from feedback

* Allow 'readonly' on getter-only auto props and get accessors

* Update spec. Improve error messages.

* readonly members warnings WIP

* Remove dead code

* Readonly members semantic checks. Warn on implicit copy of 'this'.

* Add to readonly ref readonly test

* Warn on implicit copies for get property access

* Don't warn on calls to base methods

* Add test for setter accesses in a readonly member

* Allow readonly setter accesses in readonly methods

* Test increment, compound assignment, and field-like events

* Fixes from feedback

* Add mutating readonly event accessors test

* Expand on readonly base members test

* Check for implicit copies in foreach/using/deconstruction scenarios

* Split base method tests into multiple tests for clarity

* Test remaining cases

* Fix whitespace in test

* Add ReadOnlyMethod_RefLocal

* Fixes from Fred's feedback

 Adjust base access with explicit base according to latest LDM decisions

- Restrict accessed members to immediate members of the specified type.
- Change accessibility of explicit implementations in interfaces to ‘protected’.

Fix missing using directive in unit test.

Forbid Obsolete/Deprecated on event accessors.

Give suitable error messages for Obsolete/Deprected on event accessors, and on property accessors pre-C# 8

Add further tests

* Improve the error message for an 'in' extension method so it's clear why constraining it to a struct wont help.

Simplify error message to not mention unconstrained type parameter that it doesn't apply to. (#33882)

Fixes #33847

Fixes #33684.