librustc: handle repr on structs, require it for ffi, unify with packed

As of RFC 18, struct layout is undefined. Opting into a C-compatible struct
layout is now down with #[repr(C)]. For consistency, specifying a packed
layout is now also down with #[repr(packed)]. Both can be specified.

To fix errors caused by this, just add #[repr(C)] to the structs, and change
 #[packed] to #[repr(packed)]

Closes #14309

[breaking-change]

src/test/debuginfo/c-style-enum-in-composite.rs

@@ -87,7 +87,7 @@ struct PaddedStruct {
     e: i16
 }
 
-#[packed]
+#[repr(packed)]
 struct PackedStruct {
     a: i16,
     b: AnEnum,