Currently, the lambda rewriter has an early optimization pass in
analysis that tries to find all local functions that only capture 'this'
and remove references to local functions that do the same. There are two
problems with this approach:

    1) Generally, removing information from the tree is a bad idea
    because it hurts further analysis passes that may have needed that
    information.

    2) The optimization strategy itself is very tricky and has a number
    of complex corner cases. This has lead to bugs, for example #19033.

This PR deletes the current method and adds a new optimization routine
at the end of the analysis, operating on assigned scopes and
environments rather than removing captured variable analysis. The new
optimization is as follows: if we end up with an environment containing
only 'this', the environment can be removed, all containing methods can
be moved to the top-level type, and all environments which capture the
'this' environment can instead directly capture the 'this' parameter.
This produces almost the same results as the previous optimization, but
is easier to validate as an algebraic equivalence.

The baseline changes come from the new optimization being less aggressive 
about moving functions which only capture 'this' to the top level. This
appears to be a wash -- some codegen gets slightly better, some gets
slightly worse.

Fixes #19033
Fixes #20577

* Compare lines instead of using AssertXml when validating PDBs.

AssertXml doesn't validate ordering.

* Reorder expected PDB XML nodes to match actual order

* Update to the latest Microsoft.DiaSymReader.* packages.

* Enable embedding sources to Windows PDBs

Merged on behalf of @svick 
Thanks for the contribution!

* Clean up Concord exception utilities

* Avoid dependency on IL stream in EnC local slot mapping

* Fix lambda EnC

* Tagging VB/C# Ioperation tests

* Fix the Wrong Tagging

The frame creation pass now considers scope of captured variable
introduction instead of closure creation, so the ordering can change if
two or more nested functions access variables 'outside-in', e.g.

```csharp
{
    int x = 0;
    {
        int y = 0;
        int L1() => y;

        {
             int z = 0;
             int L2() => x;
        }
    }
}
```

If we visit closures in-order in the previous example, we create frames
for L1, then L2. If we visit captured variables scopes, however, we
visit `x` (captured by L2), then `y` (captued by L1).

This is part of the effort to move as much logic from LambdaRewriter
into Analysis as possible. The more logic there is in the Rewriter, the
more difficult it is to find calculation problems until the last
possible moment, and it's also very difficult to calculate useful
information for later analysis passes, like the final debug and closure
IDs for each environment.

fixed issue in C# GetBindableSyntaxNode which return bindable node that is outside of SemanticModel.Root

* Adding ioperation tests for WhileUntilLoopStatement

* Addressing Feedback

* Removing redundant Nested tests

* Removing the empty lines

Merging on behalf of @svick 
Thanks for the contribution!