codegen_llvm: set DW_AT_accessibility

Sets the accessibility of types and fields in DWARF using
`DW_AT_accessibility` attribute.

`DW_AT_accessibility` (public/protected/private) isn't exactly right for
Rust,  but neither is `DW_AT_visibility` (local/exported/qualified), and
there's no way to set `DW_AT_visbility` in LLVM's API.

Signed-off-by: David Wood <david@davidtw.co>

tests/codegen/debug-accessibility/private-struct.rs

@@ -0,0 +1,17 @@
+// compile-flags: -C debuginfo=2
+
+#![allow(dead_code)]
+
+// Checks that visibility information is present in the debuginfo for private structs.
+
+use std::hint::black_box;
+
+struct PrivateFooStruct {
+    x: u32,
+}
+
+// CHECK: {{!.*}} = !DICompositeType(tag: DW_TAG_structure_type, name: "PrivateFooStruct"{{.*}}flags: DIFlagPrivate{{.*}})
+
+fn main() {
+    black_box(PrivateFooStruct { x: 1 });
+}