Merge recent bits from master

Clarify error message when this is returned by reference

Disable vsix check.

CR feedback on ref structs

The PDB path which is written into the PE file should respect the
`/pathmap` option passed to the compiler.  This is necessary to ensure
that PEs can be deterministic when built from different source paths.

The feature flag `pdb-path-determinism` is being kept for the time
being.  Even though it's a feature flag it still seems inappropriate to
break customers in a point release.  Deferring the removal until the
next major release to give customers time to react.  This issue tracks
removing the flag:

https://github.com/dotnet/roslyn/issues/19592

closes #9813

* 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

Reuse local rewriter to derive arguments for IHasArgumentsExpression

LambdaRewriter should only assign proxies from locals in original frame in EE

Fix bug when caret is at the end of a numeric literal

Issue #10428: Formatting of empty loops now respects CSharp spacing options

Enable debug information by default in CSI

Fix bug when caret is at the end of an argument

Fix perf regression in gotodef.

Move DocumentHighlights data types down to the 'Features' layer.

Fix FQN for CodeLens (for test discovery)

Reduce overhead from memory mapped files