Cleanup PROTOTYPE comments

* Add errors for ref-readonly signatures, readonly structs, and ref structs

* EE now working with ref-readonly

* Fix failing tests

* PR Comments

Making default type of stackalloc expression to be Span<T> when not directly in an initializer

Optimizes emit for trivial/common cases of stackalloc

Revert Microsoft.NETCore.Compilers.props to 6/8 version

Fix NETCore.Compiler Package

merge master to features/readonly-ref

* Load DSRN from alternate path if set

* Fix error reporting

Add support for the CLI invoking Roslyn without CscToolPath

Don't write files in CompilerPackage if not needed

Merge pull request #20805 from dotnet/features/non-trailing

Fixes #21889

Disabling flakey VisualBasic.BasicEditAndContinue.MultiProjectDebuggingWhereNotAllModulesAreLoaded (#21926)

* disabling flakey test

* disabling all VB ENC tests

CompilerPackage was previously being lazy and would just delete and
rewrite files as needed when the package was loaded, even if the
machine would ultimately be left in the same state at the end.
For the old project system (which didn't monitor such file changes)
this wasn't bad, but with the new project system it now reloads projects
in response to changes which can be quite expensive.

Fixes dotnet/roslyn#21838.

* fixed breaking changes markdown file

* Ban illegal assignment expressions

* Replacing conversions with a bound converted node

* Clean up

* PR comments

* Code comments

Fixed Intellisense suggestion non-viable constructor call in delegating constructor

in escape analysis receivers of readonly structs should be considered as `in` parameters.

Merge latest bits from master branch into features/non-trailing