do not allow ABI mismatches inside repr(C) types

tests/fail/function_pointers/abi_mismatch_repr_C.rs

@@ -0,0 +1,16 @@
+use std::num::*;
+
+#[repr(C)]
+struct S1(NonZeroI32);
+
+#[repr(C)]
+struct S2(i32);
+
+fn callee(_s: S2) {}
+
+fn main() {
+    let fnptr: fn(S2) = callee;
+    let fnptr: fn(S1) = unsafe { std::mem::transmute(fnptr) };
+    fnptr(S1(NonZeroI32::new(1).unwrap()));
+    //~^ ERROR: calling a function with argument of type S2 passing data of type S1
+}

tests/fail/function_pointers/abi_mismatch_repr_C.stderr

@@ -0,0 +1,17 @@
+error: Undefined Behavior: calling a function with argument of type S2 passing data of type S1
+  --> $DIR/abi_mismatch_repr_C.rs:LL:CC
+   |
+LL |     fnptr(S1(NonZeroI32::new(1).unwrap()));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ calling a function with argument of type S2 passing data of type S1
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = help: this means these two types are not *guaranteed* to be ABI-compatible across all targets
+   = help: if you think this code should be accepted anyway, please report an issue
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/abi_mismatch_repr_C.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+