Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/unsupported_elided.stderr

@@ -0,0 +1,23 @@
+error[E0726]: implicit elided lifetime not allowed here
+ --> tests/ui/unsupported_elided.rs:6:14
+  |
+6 |         type T;
+  |              ^ expected lifetime parameter
+  |
+  = note: assuming a `'static` lifetime...
+help: indicate the anonymous lifetime
+  |
+6 |         type T<'_>;
+  |               ++++
+
+error[E0106]: missing lifetime specifier
+ --> tests/ui/unsupported_elided.rs:8:24
+  |
+8 |         fn f(t: &T) -> &str;
+  |                 --     ^ expected named lifetime parameter
+  |
+  = help: this function's return type contains a borrowed value, but the signature does not say which one of `t`'s 2 lifetimes it is borrowed from
+help: consider introducing a named lifetime parameter
+  |
+8 |         fn f<'a>(t: &'a T<'a>) -> &'a str;
+  |             ++++     ++  ++++      ++