* Top-level types enumeration refactoring

* Simplify DefinitionMap

* Include namespaces in synthesized definition map

* Add SynthesizedNamespaceSymbol

* Embedded Attributes

* Test synthesized attributes

* Clean up EnC tests

* Clean up symbol matcher tests

* Ref readonly return type, in parameter symbol matching tests

* Enable using ref readonly returns, in parameters, ref structs during debugging

* More tests

* ReadOnly struct members

* Fix langauge version mismatch

* Merge fix ups

* Feedback, skip tests

Fixes #37111

Fixes #30067

* EnC LocalSlotMappingTests for C# 8.0 patterns
* PDBTest
* EditAndContinueTest

Fixes #36963

The semantic model often creates ReducedExtensionMethods for the public API,
but the creation of ReducedExtensionMethod does constraint checking, which
requires a compilation, which isn't always available. This change removes constraint
checking from all creation of ReducedExtensionMethod and adds asserts that we
always have a compilation during constraint checking.

Fixes #37021

This also adds suppressions or asserts for places in Workspace.cs that
I enabled in my previous commit.

This adds a null check to Workspace.SetCurrentSolution;
the method would have thrown but not in a clean way.