Old LLVM bugs are reportedly cropping up harder, but 3.9 seems to be OK.

Fixes rust-lang#45277.

The necessary changes were only in upstream LLVM in 4.0, but they were
for a while backported to Rust LLVM.  Now that Rust LLVM is also 4.0, we
can make the test conditional here more accurate.

While the `config.toml.example` comments say "we automatically check the
version by default," we actually didn't.  That check was badly out of
date, only allowing 3.5, 3.6, or 3.7.  This it now updated to the new
3.9 minimum requirement, and truly enabled by default.