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).
Use a macro in lang_items to remove duplication.

This should make maintenance of lang items simpler and also reduces the
line count by about 201 lines.
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.
Check crate root for docs in missing_doc lint.

Because the root module isn't actually an item, we need to do some hackish
handling of it.

Closes #10656.
auto merge of #10665 : cmr/rust/doc_lint, r=alexcrichton

Because the root module isn't actually an item, we need to do some hackish
handling of it.

Closes #10656.
auto merge of #10832 : chris-morgan/rust/let's-lop-lang-item-line-count, r=alexcrichton

This should make maintenance of lang items simpler and also reduces the
line count by about 201 lines.
Link rustllvm statically to librustc

In order to keep up to date with changes to the libraries that `llvm-config`
spits out, the dependencies to the LLVM are a dynamically generated rust file.
This file is now automatically updated whenever LLVM is updated to get kept
up-to-date.

At the same time, this cleans out some old cruft which isn't necessary in the
makefiles in terms of dependencies.

Closes #10745
Distribute a unified binary for snapshots

This means that a snapshot is purely a binary with no rust dynamic dependencies!

Closes #10744