debuginfo: Enums, generators, and async envs have generic arguments at the wrong level

[michaelwoerister]

Currently, the compiler emits debuginfo that attaches generic arguments to the type of each variant instead of at the top-level. For example, the enum

enum Foo<T> {
   Bar(T),
   Baz(u32),
}

gets described as if it was

enum Foo {
   Bar<T>(T),
   Baz<T>(u32),
}

It would be more consistent and efficient if the generic arguments were pulled into the type describing Foo.

The main obstacle to doing so is making sure that this change does not break existing tooling.

Here is some actual DWARF (for the Poll enum) to illustrate the issue:

< 4><0x000007b4>          DW_TAG_structure_type
                            DW_AT_name                  Poll<u8>
                            DW_AT_byte_size             0x00000002
                            DW_AT_alignment             0x00000001
< 5><0x000007bb>            DW_TAG_variant_part
                              DW_AT_discr                 <0x000007c0>
< 6><0x000007c0>              DW_TAG_member
                                DW_AT_type                  <0x00001335>
                                DW_AT_alignment             0x00000001
                                DW_AT_data_member_location  0
                                DW_AT_artificial            yes(1)
< 6><0x000007c7>              DW_TAG_variant
                                DW_AT_discr_value           0
< 7><0x000007c9>                DW_TAG_member
                                  DW_AT_name                  Ready
                                  DW_AT_type                  <0x000007e4>
                                  DW_AT_alignment             0x00000001
                                  DW_AT_data_member_location  0
< 6><0x000007d5>              DW_TAG_variant
                                DW_AT_discr_value           1
< 7><0x000007d7>                DW_TAG_member
                                  DW_AT_name                  Pending
                                  DW_AT_type                  <0x00000800>
                                  DW_AT_alignment             0x00000001
                                  DW_AT_data_member_location  0
< 5><0x000007e4>            DW_TAG_structure_type
                              DW_AT_name                  Ready
                              DW_AT_byte_size             0x00000002
                              DW_AT_alignment             0x00000001
< 6><0x000007eb>              DW_TAG_template_type_parameter
                                DW_AT_type                  <0x00001335>
                                DW_AT_name                  T
< 6><0x000007f4>              DW_TAG_member
                                DW_AT_name                  __0
                                DW_AT_type                  <0x00001335>
                                DW_AT_alignment             0x00000001
                                DW_AT_data_member_location  1
< 5><0x00000800>            DW_TAG_structure_type
                              DW_AT_name                  Pending
                              DW_AT_byte_size             0x00000002
                              DW_AT_alignment             0x00000001
< 6><0x00000807>              DW_TAG_template_type_parameter
                                DW_AT_type                  <0x00001335>
                                DW_AT_name                  T

There should be a single DW_TAG_template_type_parameter DIE attached to the top-level DW_TAG_structure_type instead of duplicated across the DW_TAG_structure_types for Ready and Pending.