Fixes #41682
Fixes #40326

Fixes #39731.

- Declaration of classes, structures and enums within variant interfaces is disallowed.
- Nested classes synthesized by the compiler don't have variant emitted type parameters.
- Local functions and lambdas used within a variant interface always have display class.

* Implement lambda discard parameters

* Remove duplicate tests

* Factor logic to recognize underscore tokens

* Align symbol display with other discards

* Distinguish discards in QuickInfo

* Add IsDiscard property instead of IDiscardSymbol

* Adjust IDE logic to use IsDiscard

* Avoid large tuple return

* Add test for ref/out discard parameters

* Address PR feedback and test plan ideas

* Allow in expression trees

* Support general discard parameters

* Add IDE tests

* Emit with unspeakable name

* Remove unused error code

* Only support lambdas

* Fix tests

* Address some PR feedback

* Address remaining feedback

* Address more feedback

* Fix conflict

* Update PublicAPI.Unshipped.txt

* Update PublicAPI.Unshipped.txt

* Add ChangeSignature tests

* Add SymbolCompletion tests

* Add InlineRename test

* Typo

* async -> asynchronous
* Fix up error messages for await using declarations in langver < 8.0
* Use CSharpTestBase.IAsyncDisposableDefinition for interfaces

Fixes #38571

Fixes #27060

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.