Update LLVM and jettison jit support

LLVM's JIT has been updated numerous times, and we haven't been tracking it at
all. The existing LLVM glue code no longer compiles, and the JIT isn't used for
anything currently.

This also rebases out the FixedStackSegment support which we have added to LLVM.
None of this is still in use by the compiler, and there's no need to keep this
functionality around inside of LLVM.

This is needed to unblock #10708 (where we're tripping an LLVM assertion).
Remove unused upcalls

The main one removed is rust_upcall_reset_stack_limit (continuation of #10156),
and this also removes the upcall_trace function. The was hidden behind a
`-Z trace` flag, but if you attempt to use this now you'll get a linker error
because there is no implementation of the 'upcall_trace' function. Due to this
no longer working, I decided to remove it entirely from the compiler (I'm also a
little unsure on what it did in the first place).
auto merge of #10758 : alexcrichton/rust/upgrade-llvm, r=alexcrichton

This upgrades LLVM in order to make progress on #10708, and it's also been awhile since we last upgraded!

The contentious point of this upgrade is that all JIT support has been removed because LLVM is changing it and we're not keeping up with it.