debuginfo: Refactor debuginfo generation for types

This commit
- changes names to use di_node instead of metadata
- uniformly names all functions that build new debuginfo nodes build_xyz_di_node
- renames CrateDebugContext to CodegenUnitDebugContext (which is more accurate)
- moves TypeMap and functions that work directly work with it to a new type_map module
- moves and reimplements enum related builder functions to a new enums module
- splits enum debuginfo building for the native and cpp-like cases, since they are mostly separate
- uses SmallVec instead of Vec in many places
- removes the old infrastructure for dealing with recursion cycles (create_and_register_recursive_type_forward_declaration(), RecursiveTypeDescription, set_members_of_composite_type(), MemberDescription, MemberDescriptionFactory, prepare_xyz_metadata(), etc)
- adds type_map::build_type_with_children() as a replacement for dealing with recursion cycles
- adds many (doc-)comments explaining what's going on
- changes cpp-like naming for C-Style enums so they don't get a enum$<...> name (because the NatVis visualizer does not apply to them)
- fixes detection of what is a C-style enum because some enums where classified as C-style even though they have fields
- changes the position of discriminant debuginfo node so it is consistently nested inside the top-level union instead of, sometimes, next to it

compiler/rustc_codegen_ssa/src/debuginfo/mod.rs

@@ -1,2 +1,34 @@
+use rustc_middle::ty::{self, layout::TyAndLayout};
+use rustc_target::abi::Size;
+
 // FIXME(eddyb) find a place for this (or a way to replace it).
 pub mod type_names;
+
+/// Returns true if we want to generate a DW_TAG_enumeration_type description for
+/// this instead of a DW_TAG_struct_type with DW_TAG_variant_part.
+///
+/// NOTE: This is somewhat inconsistent right now: For empty enums and enums with a single
+///       fieldless variant, we generate DW_TAG_struct_type, although a
+///       DW_TAG_enumeration_type would be a better fit.
+pub fn wants_c_like_enum_debuginfo<'tcx>(enum_type_and_layout: TyAndLayout<'tcx>) -> bool {
+    match enum_type_and_layout.ty.kind() {
+        ty::Adt(adt_def, _) => {
+            if !adt_def.is_enum() {
+                return false;
+            }
+
+            match adt_def.variants().len() {
+                0 => false,
+                1 => {
+                    // Univariant enums unless they are zero-sized
+                    enum_type_and_layout.size != Size::ZERO && adt_def.all_fields().count() == 0
+                }
+                _ => {
+                    // Enums with more than one variant if they have no fields
+                    adt_def.all_fields().count() == 0
+                }
+            }
+        }
+        _ => false,
+    }
+}