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

src/test/codegen/generator-debug.rs

@@ -22,7 +22,7 @@ fn generator_test() -> impl Generator<Yield = i32, Return = ()> {
 
 // CHECK-DAG:  [[GEN_FN:!.*]] = !DINamespace(name: "generator_test"
 // CHECK-DAG:  [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_structure_type, name: "{generator_env#0}", scope: [[GEN_FN]]
-// CHECK:      [[VARIANT:!.*]] = !DICompositeType(tag: DW_TAG_variant_part, scope: [[GEN_FN]],
+// CHECK:      [[VARIANT:!.*]] = !DICompositeType(tag: DW_TAG_variant_part, scope: [[GEN]],
 // CHECK-NOT:  flags: DIFlagArtificial
 // CHECK-SAME: discriminator: [[DISC:![0-9]*]]
 // CHECK:      {{!.*}} = !DIDerivedType(tag: DW_TAG_member, name: "0", scope: [[VARIANT]],
@@ -54,7 +54,7 @@ fn generator_test() -> impl Generator<Yield = i32, Return = ()> {
 // CHECK:      {{!.*}} = !DIDerivedType(tag: DW_TAG_member, name: "s", scope: [[S1]]
 // CHECK-NOT:  flags: DIFlagArtificial
 // CHECK-SAME: )
-// CHECK:      [[DISC]] = !DIDerivedType(tag: DW_TAG_member, name: "__state", scope: [[GEN_FN]],
+// CHECK:      [[DISC]] = !DIDerivedType(tag: DW_TAG_member, name: "__state", scope: [[GEN]],
 // CHECK-SAME: flags: DIFlagArtificial
 
 fn main() {