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

Add a regression test for https://github.com/dotnet/roslyn/issues/21766

Conflicts:
	src/Compilers/CSharp/Portable/Errors/MessageID.cs
	src/Compilers/VisualBasic/Portable/Parser/ParserFeature.vb

Move Roslyn to .NET Core 2.0 SDK

Fixes:#21880

Val-escape for a struct field should defer to _val_ escape of the receiver

Bug fixes and testing for completion in XML documentation comments

Change Microbuild to no longer be forced to use the MSBuild that
Microbuild provides. Doing so means we rely on them to update their
machines when we need updates to MSBuild.

Instead just use our normal mechanisms for discovering MSBuild. This
doesn't depend on machine state.

https://github.com/dotnet/sdk/issues/1552