Add test to verify arg size mismatch throws UB

Also: bump up `rust-version`

rust-version

@@ -1 +1 @@
-14b15521c52549ebbb113173b4abecd124b5a823
+e83f7563495dbe2629b0cbc738afb0808c4482e1

tests/compile-fail/shim_arg_size.rs

@@ -0,0 +1,15 @@
+#![feature(rustc_private)]
+
+extern crate libc;
+
+// error-pattern: scalar size mismatch
+fn main() {
+    extern "C" {
+        fn malloc(size: u32) -> *mut std::ffi::c_void;
+    }
+
+    unsafe {
+        let p1 = malloc(42);
+        libc::free(p1);
+    };
+}