Auto merge of #34743 - badboy:llvm-upgrade, r=eddyb

LLVM upgrade

As discussed in https://internals.rust-lang.org/t/need-help-with-emscripten-port/3154/46 I'm trying to update the used LLVM checkout in Rust.

I basically took @shepmaster's code and applied it on top (though I did the commits manually, the [original commits have better descriptions](https://github.com/rust-lang/rust/compare/master...avr-rust:avr-support).

With these changes I was able to build rustc. `make check` throws one last error on `run-pass/issue-28950.rs`. Output: https://gist.github.com/badboy/bcdd3bbde260860b6159aa49070a9052

I took the metadata changes as is and they seem to work, though it now uses the module in another step. I'm not sure if this is the best and correct way.

Things to do:

* [x] ~~Make `run-pass/issue-28950.rs` pass~~ unrelated
* [x] Find out how the `PositionIndependentExecutable` setting is now used
* [x] Is the `llvm::legacy` still the right way to do these things?

cc @brson @alexcrichton

configure

@@ -1020,6 +1020,12 @@ then
             err "bad LLVM version: $LLVM_VERSION, need >=3.7"
             ;;
     esac
+
+    if "$CFG_LLVM_ROOT/bin/llvm-mc" -help | grep -- "-relocation-model"; then
+        msg "found older llvm-mc"
+        CFG_LLVM_MC_HAS_RELOCATION_MODEL=1
+        putvar CFG_LLVM_MC_HAS_RELOCATION_MODEL
+    fi
 fi
 
 # Even when the user overrides the choice of CC, still try to detect