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/allocator.rs

@@ -140,8 +140,8 @@ pub(crate) unsafe fn codegen(
     llvm::LLVMDisposeBuilder(llbuilder);
 
     if tcx.sess.opts.debuginfo != DebugInfo::None {
-        let dbg_cx = debuginfo::CrateDebugContext::new(llmod);
-        debuginfo::metadata::compile_unit_metadata(tcx, module_name, &dbg_cx);
+        let dbg_cx = debuginfo::CodegenUnitDebugContext::new(llmod);
+        debuginfo::metadata::build_compile_unit_di_node(tcx, module_name, &dbg_cx);
         dbg_cx.finalize(tcx.sess);
     }
 }