[Typed throws] Workaround for LLVM miscompile of unused swifterror, add executable test

[kubamracek]

While experimenting with typed throws, I discovered that on x86_64 and -Onone only, the IR that typed throws generates (which is identical to IR on arm64!) ends up getting lowered in a wrong way and the execution of the program ends up taking the wrong branch, this is demonstrated by the testcase added in this PR -- the "this cannot happen" call actually happens. I don't have the ability to diagnose the LLVM problem further, and so instead this PR adds a workaround to enable executable testing of typed throws. This PR also adds a simple executable test.

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The following IR demonstrates the problem. When the following is built for x86_64 under -O0…

target triple = "x86_64-apple-macosx14.0.0"
declare swiftcc i64 @throwing(ptr swiftself %0, ptr noalias nocapture swifterror dereferenceable(8) %1)
define swiftcc void @catching() {
entry:
  %swifterror = alloca swifterror ptr, align 8
  store ptr null, ptr %swifterror, align 8
  %0 = call swiftcc i64 @throwing(ptr swiftself undef, ptr noalias nocapture swifterror dereferenceable(8) %swifterror)
  %1 = load ptr, ptr %swifterror, align 8
  %2 = icmp ne ptr %1, null
  br i1 %2, label %catch, label %continue
catch:
  ; %dummy_use = ptrtoint ptr %1 to i64    ; UNCOMMENT THIS TO GET CORRECT OUTPUT
  br label %ret
continue:
  br label %ret
ret:
  ret void
}

…it produces this machine code:

] ../llvm-macosx-arm64/bin/llc -O0 x86.ll -o -
_catching:                              ## @catching
	pushq	%r13
	pushq	%r12
	pushq	%rax
	xorl	%eax, %eax
	movl	%eax, %r12d
	callq	_throwing
	cmpq	$0, (%rsp)
	je	LBB0_2

Notice that the cmp is comparing (%rsp), which is a miscompile -- swifterror is expected to be in r12. Interestingly, this only happens if there is no use of %1 in the catch block in the IR, and even a dummy unused instruction that just ends up using %1 suddenly makes the output correct.

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test Windows platform