Prevent embedding of non-abstract interface members.

Closes #35852. Closes #35911.

Fixes #7398
Fixes #37572
Relates to https://github.com/aspnet/Razor/issues/614 and many other customer accommodations that are no longer needed.

History of this bug:  When constant folding a long sequence of string concatenations, there is an intermediate constant value for every left-hand operand.  So the total memory consumed to compute the whole concatenation was *O(n^2)*.  The compiler would simply perform this work and eventually run out of memory, simply crashing with no useful diagnostic.  Later, the concatenation implementation was instrumented so it would detect when it was likely to run out of memory soon, and would instead report a diagnostic at the last step. See https://github.com/dotnet/roslyn/commit/f177077665a5bef862014f1898b961d1759c9248.  Test `Bug529600()` was added to demonstrate that we produced a diagnostic.  However, the compiler still consumed *O(n^2)* memory for the concatenation and this test used to consume so much memory that it would cause other tests running in parallel to fail because they might not have enough memory to succeed.  So the test was disabled and eventually removed.  The compiler would still crash with programs containing large string concatenations, or consume huge amounts of memory and take a long time before reporting a diagnostic, so the underlying problem had not been addressed.

Here we have revised the implementation of constant folding string concatenations so that it requires *O(n)* memory and remove the old instrumentation.  As a consequence the test `Bug529600()` now runs very quickly and does not consume gobs of memory.

Fixes #33821 

Implements https://github.com/dotnet/csharplang/issues/2389
Fixes #35958

Note that the switch expression conversion is not a standard implicit conversion.  Specifically, it should not be considered as a valid implicit conversion as input to a user-defined conversion, as that could permit two chained user-defined conversions.

Also changes binding of switch expression to ECMA so it can interact properly with switch expressions.
Fixes #14965

Fixes #37111

Support nullable variance in interface and partial method implementation.

Closes #35227.

* Add optional [warnings | annotations] keyword to nullable directive

* pragma warning <setting> nullable -> nullable <setting> warnings

* Implement /nowarn:nullable

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

* Update dependencies from https://github.com/dotnet/arcade build 20190503.8

- Microsoft.DotNet.Arcade.Sdk - 1.0.0-beta.19253.8

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- Microsoft.DotNet.Arcade.Sdk - 1.0.0-beta.19270.1

* 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