[llvm] [AsmPrinter][DebugNames] Implement DW_IDX_parent entries (PR #77457)

[Greg Clayton via llvm-commits]

clayborg wrote:

> added test as suggested by @ayermolo I believe we're looking good to merge now?

The above forward declarations should _not_ end up in the name lookup as that will violate the DWARF spec. It is ok for an entry to be in the list of entries, but any foward declarations should not be able to be looked up by name. It _is_ ok for an entry to be in there for the parent index stuff, but we don't want to lookup MyStruct2 and find either entries from:
```
0x00000029:   DW_TAG_namespace
                DW_AT_name      ("MyNamespace")

0x0000002b:     DW_TAG_structure_type
                  DW_AT_declaration     (true)
                  DW_AT_signature       (0x8b108ad18e93cb95)

0x00000034:     DW_TAG_structure_type
                  DW_AT_declaration     (true)
                  DW_AT_signature       (0x690ca39e0eeaec8f)

0x0000003d:     NULL
```


https://github.com/llvm/llvm-project/pull/77457