librustc: Forbid transmute from being called on types whose size is

only known post-monomorphization, and report `transmute` errors before
the code is generated for that `transmute`.

This can break code that looked like:

    unsafe fn f<T>(x: T) {
        let y: int = transmute(x);
    }

Change such code to take a type parameter that has the same size as the
type being transmuted to.

Closes #12898.

[breaking-change]

src/libstd/dynamic_lib.rs

@@ -134,7 +134,7 @@ impl DynamicLibrary {
     }
 
     /// Access the value at the symbol of the dynamic library
-    pub unsafe fn symbol<T>(&self, symbol: &str) -> Result<T, String> {
+    pub unsafe fn symbol<T>(&self, symbol: &str) -> Result<*T, String> {
         // This function should have a lifetime constraint of 'a on
         // T but that feature is still unimplemented