Integrate LLVM at 836201f (#21148)

Integrate LLVM at llvm/llvm-project@836201f.

This catches up from ~1 week ago to ~present, but in exchange for that,
we carry over the existing revert and add 2 more reverts of a pair of
related commits each:

Carried-over reverts:
*
llvm/llvm-project@e4de74b.
I don't understand it, but Rob discovered that it unblocked the previous
integrate.

New reverts:
*
llvm/llvm-project@ee070d0
and
llvm/llvm-project@a1c2a71:
after making the necessary C++ fixes to unbreak the build
(https://gist.github.com/bjacob/a132e70ea0f7ceba4554f352a45bcd1c), these
break bufferization and eliminate-empty-tensors tests that I don't
understand.
* Corresponding Bazel changes also reverted:
llvm/llvm-project@2a41350,
llvm/llvm-project@8fc20bf.
*
llvm/llvm-project@eb694b2
and
llvm/llvm-project@e33f13b
: these break `arith.trunci` with destination type `i1`, which occurs at
least in the `jit_globals.mlir` test.

Signed-off-by: Benoit Jacob <[email protected]>

Author: bjacob