Merge master to dev15.6

Fixes #18948

See #19682

* Fixes crash in `is` operator used with tuples
Fixes #19310

* Matching an expression of type `byte?` with a pattern of type `int` should not be allowed
* Add documentation for the breaking change matching `int?` against the pattern `long x`
Fixes #19151

* Unify subsumption implementations
Fixes #19273

* Greatly improved code gen for value type pattern match
Fixes #16129
Fixes #18944
Fixes #14252

* Add test confirming that #19122 is fixed.
Fixes #19122

Fix for deconstruction-assignment to write-only property.

Don't show completionlist completion after dot

Naming Styles Accessibility

Merge master to dev15.6

Revert some accidental code style changes

Merge master into 15.6

Skip user-defined conversions when checking extension method 'this' argument

bypass command line argument passing if no command line parser is available

Include overflow checks in tuple conversions

Clarify error message when this is returned by reference

Disable vsix check.

Use static methods in CSharpSyntaxGenerator where possible

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

Reuse local rewriter to derive arguments for IHasArgumentsExpression

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