Add support for LLVM 18

[elliottslaughter]

Currently passing all but 5 tests on macOS x86:

99% tests passed, 5 tests failed out of 550

Total Test time (real) =  52.38 sec

The following tests FAILED:
	 65 - class.t (SEGFAULT)
	 67 - class3.t (SEGFAULT)
	 68 - class4.t (SEGFAULT)
	 84 - constant2.t (SEGFAULT)
	 85 - constantinits.t (SEGFAULT)

[elliottslaughter]

The crashes all go away when I build LLVM and Terra with Address Sanitizer. I'm not quite sure what that means right now. I'm trying a couple more options to make sure this isn't the result of unrelated build flag changes. Unfortunately it's going to make debugging this a bit more difficult.

[elliottslaughter]

The crashing tests were a result of not clearing out a function analysis manager when deleting a function. This also fixes #644 and probably #654.

[elliottslaughter]

FreeBSD CI services are still down so I won't attempt to spin up an LLVM 18 test there.