Windows: LLVM 12+ fail in JIT

[elliottslaughter]

Found this while working on #539. Windows build seems to have trouble with LLVM 12-14, but works in LLVM 11. LLVM and Terra all have identical build environments and settings (just built all the LLVM binaries today).

Sample error message looks like:

llvm: JIT has not been linked in.

This string does not originate from Terra.

For posterity I'm attaching the log from an AppVeyor run that demonstrates the problem here: log.txt

Edit: confirmed that the problem still occurs in LLVM 14.