Test/fix scoping rules for pattern locals in field/property initializers, parameter default values and attributes.

Fixes #8816.

Fixes #8810.
Related to #8816.

Fixes #9072 #9073 #9079 #9080

Fixes #8809.
Related to #9210.

Do not allow a local function declaration as an embedded statement.
Handle deep expressions better in the pattern variable finder.

Modify pattern matching tests to use new "patternsExperimental" flag for
those pattern-matching features not yet ready for prime time.

Related to #8817.

- Check for name conflicts for a local declared with [let <identifier> = …] statement.
- Create PatternVariableBinder for ReturnStatementSyntax.
- Create PatternVariableBinder for ThrowStatementSyntax.
- Create BlockBinder for a [let] statement used as an embedded statement.
- Add SemanticModel.GetDeclaredSymbol API for LetStatementSyntax.
- Test scope of pattern variable declared in [let], [return] and [throw] statements.

Related to # 8817.

reverse-engineering the relationship between positions and properties
by examining the names of constructor parameters.
See also #8415

Also add tests for scoping of variables declared within expression statements by using DeclarationPatternSyntax.

Related to #8817.

Implemented the match expression. #5154
Implemented the throw expression. #5143

Added tests for switch, foreach.

Added support and tests for lambdas and local functions, but it doesn't work so tests are skipped.

Implementation and test for pattern matching to an unconstrained type parameter.

if (o is Point { X is int x, Y is int y }) ...

Fix a bug in value pattern lowering.

dangling local declarations in a speculative semantic model,
marking them
        [Fact(Skip = "https://github.com/dotnet/roslyn/issues/206")]

1. Do not parse as a declaration pattern when an identifier might be a contextual identifier with its usual meaning. This occurs in query expressions.
2. Implement flow analysis for declaration patterns: any pattern variables are *definitely assigned when true* for the enclosing `is` pattern matching expression
3. Implement lowering
4. Punch through; one test case works end-to-end.