Enable opaque pointers in LLVM 16

[elliottslaughter]

If this PR works, then we'll be past the transition and will be able to complete #553.

[elliottslaughter]

I'm hitting a 96% pass rate now (LLVM 16):

96% tests passed, 24 tests failed out of 550

Total Test time (real) =  17.21 sec

The following tests FAILED:
	 19 - array.t (SEGFAULT)
	 37 - blocking2-fixed.t (SEGFAULT)
	 38 - blocking2.t (SEGFAULT)
	 39 - blocking3.t (SEGFAULT)
	 41 - bounce.t (SEGFAULT)
	 53 - call.t (SEGFAULT)
	 59 - cconv.t (SEGFAULT)
	 65 - class.t (SEGFAULT)
	 70 - class6.t (Subprocess aborted)
	 90 - coverage3.t (SEGFAULT)
	121 - defercond.t (SEGFAULT)
	129 - diffuse.t (SEGFAULT)
	334 - let2.t (Subprocess aborted)
	348 - luaterramethod.t (SEGFAULT)
	362 - method.t (SEGFAULT)
	401 - overloadmethod.t (SEGFAULT)
	404 - overloadproduct.t (SEGFAULT)
	411 - pattern.t (SEGFAULT)
	452 - scc.t (Subprocess aborted)
	478 - ssimple.t (SEGFAULT)
	482 - stencil.t (SEGFAULT)
	521 - teststd.t (SEGFAULT)
	522 - testvector.t (SEGFAULT)
	523 - torturechain.t (SEGFAULT)

Bizarrely, I had to disable deletion of function bodies when running with opaque pointers or else nearly any test with two or more functions would fail (something like 85% of the test suite). Worked around for now, but I wonder if that's an LLVM bug.

[elliottslaughter]

Superseded by #643.