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_llvm/src/debuginfo/utils.rs

@@ -1,7 +1,7 @@
 // Utility Functions.
 
 use super::namespace::item_namespace;
-use super::CrateDebugContext;
+use super::CodegenUnitDebugContext;
 
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::layout::{HasParamEnv, LayoutOf};
@@ -35,7 +35,7 @@ pub fn create_DIArray<'ll>(
 #[inline]
 pub fn debug_context<'a, 'll, 'tcx>(
     cx: &'a CodegenCx<'ll, 'tcx>,
-) -> &'a CrateDebugContext<'ll, 'tcx> {
+) -> &'a CodegenUnitDebugContext<'ll, 'tcx> {
     cx.dbg_cx.as_ref().unwrap()
 }