Closes #25560

* updated matrix for unreachable code case

* more unreachable tests

* updated comments

* add support for local function

* Rename Foo to Goo

* Rename ifoo to igoo

* Add back BOMs

By design, when data flow analyzes local functions it does so devoid of
surrounding context and records possibly unassigned variables rather
than reporting a diagnostic. Thus, all captured variables are expected
to be marked unassigned during data flow analysis without reporting a
diagnostic.

However, reporting a diagnostic is not the only side effect of running
data flow analysis. By calling virtual methods data flow analysis
informs derived types of unassigned variables. While the diagnostic is
suppressed and recorded, currently the virtual method is still called.
This has negative consequences for region analysis, which considers
these calls as indications of variables flowing out of the given
region, which may not be the case for local functions.

This PR changes data flow analysis to only call the ReportUnassigned
virtual method if not inside a local function, excluding local functions
from all unassignment reporting.

Fixes #17165, #18347

Closes #19136

Fixes #17474

Temporarily marks data flow analysis over a region which contains local
functions as "failed." The current implementation is incorrect, so data
flow analysis should mark the result as bad rather than knowingly give
an incorrect result.

This change also causes extract method to emit a warning when extracting
over a region which involves local functions.

Fixes #15532, #16116, #8719, #8718, #8717

These tests were all using WpfFact, but not actually doing anything
that required either WPF or an STA thread. We plan to enable
parallelization of Facts (but not WpfFacts), and this ensures we can
parallelize as many tests as possible.

...until we have time to think about which are the most valuable and how
they should interact with previous submissions.

This reverts 99317ad0.

Adding unit tests for Introduce variable and Extract Method in Nested
Collection Initializer cases.
The bug no longer reproes. This change is test only.