Fixes #18348

* IDE Changes

* tests

* PR comments

* Don't report modifier errors while parsing.

* Fix bad merge.

* restore code.

* Fix test.

* Remove old resource

* Alternative way to do partial checking.

* Alternative way to do partial checking.

* Simplify formatting.

* Restore formatting.

* Restore formatting.

* Restore formatting.

* Restore formatting.

* Restore formatting.

* Give a proper message.

* Update error positions.

* Update error positions.

* Update error positions.

* Actually report error on bad modifier.

LocalFunctionTypeParameterSymbol isn't necessary as long as the functionality around type constraints can be moved into a shared base class between ordinary method symbols and local function symbols.
In fact, it would be very useful to have a shared base symbol aside from MethodSymbol between local functions, lambdas, and other source method symbols. To this end, I've renamed SourceMemberMethodSymbol to SourceOrdinaryMethodSymbol (since it is just used for ordinary member-level methods), SourceMethodSymbol to SourceMemberMethodSymbol (since it is only used as a base class for member-level method-like functions like ordinary methods, constructors, property accessors, et al.), and added a new abstract class SourceMethodSymbol that is the root of both LocalFunctionSymbol and SourceMemberMethodSymbol. This lets us unify all source method-like symbols under a single base class, SourceMethodSymbol.
The first three commits perform the refactoring, while the last commit actually removes LocalFunctionTypeParameterSymbol and moves the necessary functionality into SourceMethodSymbol. I expect more functionality to be moved over in subsequent PRs.

Fixes #17244

Fixes #19950

* Parse ref/readonly-ref extension methods

* Address PR Comments

* Fix break

* More PR Comments

* Fix break

* Error CS0503 shows the correct modifier target

* Call this.DeclaringCompilation.EnsureReadOnlyAttributeExists() when needed

* Promote EmbeddedAttribute to be used by both compilers

* Generate Embedded attributes on PE Module Builder

* Thread PEModuleBuilder through Symbol.AddSynthesizedAttributes()

* Fix failing tests

* Base case passing

* Refactor ReadOnlyAttribute to IsReadOnlyAttribute

* Fix build break

* More Tests

* Clean up

* PR Comments #1

PR Comments #3

PR Comments #4

PR Comments #5

PR Comments #6

PR Comments # 7

* Handle NoPIA

Added more tests

* Lambdas and Local functions no longer generate attributes in symbols

Added tests for constructors and operators

Signal need for synthesized attributes in local rewriter for lambdas an local functions

Clean up

* Clean up

* More PR Feedback

* Adding more tests

* More PR Comments

* More tests for explicit interface implementations

* Latest PR Comments

* Moved EnsureIsReadOnlyAttributeExists to AfterAddingTypeMembersChecks

* Fix failing tests

These are mostly the checks dealing with stack-only nature of the Span. With minor difference they follow the same logic as the existing checks for types such as TypedReference.

==== Not in this change:
Defining and detecting span-like types is NYI. For now we just treat any type named "System.Span" and "System.ReadonlySpan" as span-like. This will change.

Some of the checks result in somewhat generaic messages and happen at emit phase. That was ok when the failures were supposed to be rare.
Error clarity is not the goal of this change, but we will examone what errors should say and whether they should be moved to an earlier phase.

Make local function default parameter value binding lazy

Current strict binding can cause circularity problems when local
functions are referenced. This change causes local functions to use lazy
default parameter binding, similar to methods, and then forces their
construction when diagnostics are requested for the local function.

This also requires a mechanism for recording declaration diagnostics
outside of adding to the compilation's DeclarationDiagnostics. A new
type, DeclarationDiagnosticStore is introduced as an abstraction to
store declaration diagnostics on either the compilation or in a local
DiagnosticBag, depending on the needs of the symbol storing diagnostics.

Fixes #16451, #17293

* Move more errors out of the parser.

* PR feedback.

* Move to a single diagnostic.

* Add period.

* Update text.

* Update tests.

* Fix test.

Internal APIs are aligned with these public APIs.
VB Symbol Display for ByRef returns is adjusted according to the latest design decision.

This change fixes the following issues in C# compiler: #8948 and #14453.

  (e.g. ref local variable declaration)
We introduce a "ref expression" for use in expressions
  (e.g. right-hand-side of a ref variable initialization)
Also improve the diagnostic for local functions.
Fixes #12749, #12580

[Based on features/async-return with tests ported from that branch.]

Fixes #7477

Fixes #7506

Fixes #7446