Fix bug in 'this' optimization for classes

[agocke]

In the introduction of the new 'this' optimization routine,
one of the things which was changed was to treat 'this' more
like a formal parameter of the method, as opposed to a variable
living in an implicit, higher scope. This has some advantages
in simplicity for analysis, but created a problem when it came
to proxies. The current analysis builds the proxy list by walking
the tree and finding all captured variables and adding them
to the proxy dictionary keyed by the original variable symbol.
For instance, if a local variable is captured to a field, during
rewriting it will be added to the proxy list as (original symbol,
hoisted field). Since most symbols are only ever captured to a
single replacement field, this usually works fine -- all proxies
can exist side-by-side in the proxy list since there is no intersection.

However, this is not true for captured environment pointers. When
a new environment is introduced, a local will be created to point
to that environment. That local may itself be captured by nested
variables, creating a linked list from nested scopes to parent scope.
Most notably, multiple nested environments may capture the same
environment pointer in different hoisted fields. This means that
the proxies dictionary cannot hold all mappings at once, since the
mapping for a given captured environment pointer will depend on the
current scope.

The following code exhibits this problem

using System;
class C
{
    int _x;
    void M()
    {
        {
            int y = 0;
            Func<int> f1 = () => _x + y; // Hoists 'this' to field
        }
        {
            int y = 0;
            Func<int> f2 = () => _x + y; // Hoists 'this' to different field
        }
    }
}

The current code actually accounts for this already by adding a
captured environment pointer to the proxy list on a nested scope's
introduction, and removing it upon leaving that scope.

By changing 'this' to be treated like a formal parameter, I
circumvented this logic, introducing a bug. When two scopes tried
to capture the 'this' pointer, the compiler crashed due to trying
to add two mappings to the same key.

This change fixes this problem by treating the 'this' parameter
like an environment pointer for the purposes of capturing and
hoisting. It's possible that we want to treat it like a formal
parameter, but if so it's probably better to treat all captured
environment pointers the same way and introduce a scope-aware
notion of proxies, rather than having a global dictionary.

Fixes #21506

[agocke]

cc @tmat

[khyperia]

LGTM. I think that in-person discussion on "the philosophy of how capturing works" (😛) would be good to do soon.

[agocke]

ping @jaredpar @VSadov for review

This is blocking insertion, so it needs to go in ASAP.

[VSadov]

[VSadov]

LGTM