Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/rust_pinned.stderr

@@ -0,0 +1,17 @@
+error[E0277]: `PhantomPinned` cannot be unpinned
+  --> tests/ui/rust_pinned.rs:6:14
+   |
+6  |         type Pinned;
+   |              ^^^^^^ within `Pinned`, the trait `Unpin` is not implemented for `PhantomPinned`
+   |
+   = note: consider using `Box::pin`
+note: required because it appears within the type `Pinned`
+  --> tests/ui/rust_pinned.rs:10:12
+   |
+10 | pub struct Pinned {
+   |            ^^^^^^
+note: required by a bound in `__AssertUnpin`
+  --> tests/ui/rust_pinned.rs:6:9
+   |
+6  |         type Pinned;
+   |         ^^^^^^^^^^^^ required by this bound in `__AssertUnpin`