[ASDAlexander77]

Summary

A using inside a catch clause did not work on Windows:

try { throw 1; } catch (o) {
    try { using r = new Res(); ... } catch (e: number) { ... }
}

shape	Windows on main
no try around the catch	Win32ExceptionPass crashed the compiler (the JIT at every level)
a try around the catch	ahead-of-time compilation failed LLVM's verifier ("Unwind edges out of a catch must have the same unwind dest as the parent catchswitch")

Win32ExceptionPass scanned a function's instructions in order with one current region, and made every pad within none. A try inside a catch got by because a throw in it ended the enclosing catch first, so the nested try ran outside the catch's funclet. A using breaks that, because its first call is inside the catch, before any throw. The scan then took the using's cleanup pad for the end of the catch. That left the catch with no end at all (the crash), and gave the cleanup pad one edge from inside the catch funclet and one from outside it (the verifier error).

Pads now nest:

• Tagging (TryOpLowering, LandingPadOpLowering). TryOpLowering knows which catch clause a try sits in. On Windows it tags each landing pad with its catch pad's id and the id of the catch it is nested in: the nearest try around it that holds it in its catch clause, looking through any try bodies in between. LandingPadOpLowering emits the tag as a call to ts.internal.eh_pad right after the landingpad.
• Nesting (Win32ExceptionPass). The pass reads and removes the tag, and scans with a stack of open regions. A nested region's code sits in the middle of the enclosing catch's code, and the enclosing catch carries on when the nested one ends. An end marker right after a nested catch ends the enclosing one. Nested pads are created within the enclosing catchpad. A catchswitch unwinds to the first invoke that leaves its region, never into its own descendants.

The pass could not work out the nesting from the IR alone, so the tag is required. A pad reached from a catch is either nested in it or is that catch's exit, and the IR looks the same in both cases. The old "a throw in a nested try ends the catch first" shortcut is now Linux-only.

This is the Windows gap listed as not fixed in #359.

Verification

• 00try_mismatch_in_catch_clause_using.ts now runs on every platform (compile, jit, and the rc/none corpus). Before this PR it was Linux-only. The no-try-around case that crashed the compiler is added to it.

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