* Refactor tuple implementation

* Tweaks

* Address PR feedback

* Pass tuple data in constructors

* Address more feedback

* Tweak comparison

* Restore constructWithTypeParameters flag to avoid overflow

* Assert that tuple parameter matches TupleUnderlyingType somewhat

* Rename to TupleData. Address some IDE feedback

* Remaining TODO2's

* Fix last IDE test!

* Factor array comparison

* Tweak WithTupleData and EqualsIgnoringTupleUnderlyingType

* Expand a test

* Remove duplicate line

* Remove an EE test for badly formed ValueTuple type

* PR feedback from Neal

* Restore an IDE test without ValueTuple

* Rename constants

* Restore formatting

Closes #25780

This change also separates implementation of ISymbol interfaces from internal symbols in C# compiler.

stackalloc is affected by the locals init flag, but the locals init
flag is only present if the fat IL header is used, which is not
preferred if the method is small and has no locals. S.R.M added
a parameter to allow callers to indicate that stackalloc is present
and thus localsinit should be respected when deciding to use the
fat or tiny IL header. This change tracks whether there is a stackalloc
while doing codegen and plumbs the result into S.R.M.

This flags uses of `Assert.Equal / NotEqual` where the argument is
`true` or `false`.

This enforces the use of `Assert.Null` or `Assert.NotNull`

* Require /unsafe flag for SkipLocalsInitAttribute

* Use Unsafe

Fixes #29819

Changes based on code review. Add more tests for obsolete on accessors, and Don't report 2 diagnostics for obsolete on an event in C# 7.

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

- Rename `TypeSymbolWithAnnotations` to `TypeWithAnnotations`
- Rename its type field from `TypeSymbol` to `Type`
- Fields of Symbols that are of type `TypeWithAnnotations` that are currently named `Type` would be renamed `TypeWithAnnotations`
Fixes #33736

Add unit test to confirm that an obsolete accessor does not suppress warnings from an obsolete return type

These two PRs were green but broke when built together:

- https://github.com/dotnet/roslyn/pull/28652
- https://github.com/dotnet/roslyn/pull/28420

Skip AttributeTests_WellKnownAttributes.ObsoleteOnVirtual_OnBase on Spanish while investigating failure.

* Produce errors on emitting attributes using default constructor of structs

* PR Comments