When the build is configured with `[rust] rpath = false`, we need to set
`LD_LIBRARY_PATH` (or equivalent) to what would have been the `RPATH`,
so the compiler can find its own libraries. The old `tools.mk` code has
this environment prefixed in the `$(BARE_RUSTC)` variable, so we just
need to wire up something similar for run-make v2.

This is now set while building each `rmake.rs` itself, as well as in the
`rust-make-support` helpers for `rustc` and `rustdoc` commands. This is
also available in a `set_host_rpath` function for manual commands, like
in the `compiler-builtins` test.

…OperandRef` does)

… it actually used

also allows to drop measureme dep on rustc_middle

remove some things that do not need to be

…jieyouxu

Set the host library path in run-make v2

When the build is configured with `[rust] rpath = false`, we need to set
`LD_LIBRARY_PATH` (or equivalent) to what would have been the `RPATH`,
so the compiler can find its own libraries. The old `tools.mk` code has
this environment prefixed in the `$(BARE_RUSTC)` variable, so we just
need to wire up something similar for run-make v2.

This is now set while building each `rmake.rs` itself, as well as in the
`rust-make-support` helpers for `rustc` and `rustdoc` commands. This is
also available in a `set_host_rpath` function for manual commands, like
in the `compiler-builtins` test.

Make `PlaceRef` and `OperandValue::Ref` share a common `PlaceValue` type

Both `PlaceRef` and `OperandValue::Ref` need the triple of the backend pointer immediate, the optional backend metadata for DSTs, and the actual alignment of the place (since it can differ from the ABI alignment).

This PR introduces a new `PlaceValue` type for those three values, leaving [`PlaceRef`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/place/struct.PlaceRef.html) with the `TyAndLayout` and a `PlaceValue`, just like how [`OperandRef`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/mir/operand/struct.OperandRef.html) is a `TyAndLayout` and an `OperandValue`.

This means that various places that use `Ref`s as places can just pass the `PlaceValue` along, like in the below excerpt from the diff:
```diff
        match operand.val {
-            OperandValue::Ref(ptr, meta, align) => {
-                debug_assert_eq!(meta, None);
+            OperandValue::Ref(source_place_val) => {
+                debug_assert_eq!(source_place_val.llextra, None);
                debug_assert!(matches!(operand_kind, OperandValueKind::Ref));
-                let fake_place = PlaceRef::new_sized_aligned(ptr, cast, align);
+                let fake_place = PlaceRef { val: source_place_val, layout: cast };
                Some(bx.load_operand(fake_place).val)
            }
```

There's more refactoring that I'd like to do after this, but I wanted to stop the PR here where it's hopefully easy (albeit probably not quick) to review since I tried to keep every change line-by-line clear.  (Most are just adding `.val` to get to a field.)

You can also go commit-at-a-time if you'd like.  Each passed tidy and the codegen tests on my machine (though I didn't run the cg_gcc ones).

move QueryKeyStringCache from rustc_middle to rustc_query_impl, where it actually used

Also allows to drop measureme dep on rustc_middle.

…Amanieu

Move rare overflow error to a cold function

`scoped.spawn()` generates unnecessary inlined panic-formatting code for a branch that will never be taken.

linker: Avoid some allocations in search directory iteration

This is more a cleanup than actual optimization.

Fix revisions syntax in cfg(ub_checks) test

`//@ revisions YES NO` doesn't do anything without the `:`.  Thanks for pointing this out to me.

r? jieyouxu