Pack expansion closures, part 6

[slavapestov]

Follow-up to #73685.

This implements support for autoclosures, closures and local functions nested within a pack iteration for loop, in what I believe is full generality.

Some combinations of closures also work with pack expansion expressions, but not all; this part needs more work in the solver.

General idea:

• Capture analysis records references to ElementArchetypeTypes that are defined outside of the scope of the current function.
• SIL type lowering extends the generic signature of the closure or local function with new generic parameters corresponding to the captured local archetypes.
• Interface types of AST declarations in local context may contain element archetypes; SIL type lowering maps them to interface types in the extended signature.
• SILGen lowers the code using the old generic signature's primary archetypes and local archetypes, which are out of scope, as before. A post-pass rewrites SIL instructions, replacing the old primary archetypes and local archetypes with primary archetypes for the new signature.
• SILGen rewrites substitution maps when calling a closure, forwarding substitutions for any local archetypes it captures.

When a closure doesn't capture any local archetypes, the extended generic signature is the same as the original generic signature, and we skip the substitution map transform, etc.

Fixes #66917.
Fixes #69947.
Fixes rdar://113505724.
Fixes rdar://122293832.
Fixes rdar://124329076.

[slavapestov]

@swift-ci Please smoke test

[simanerush]

Looks like this also resolves #73551 🙂

[slavapestov]

@swift-ci Please test

[slavapestov]

@swift-ci Please test source compatibility