[DebugInfo] Implement TAG_label entries for debug_names (#71724)

The DWARF 5 specification says that:

> The name index must contain an entry for each debugging information
entry that
> defines a named [...] label [...].

The verifier currently verifies this, but the AsmPrinter does not add
entries for TAG_labels in debug_names. This patch addresses the issue by
ensuring we add labels in the accelerator tables once we have a fully
completed DIE for the TAG_label entry.

We also respect the spec as follows:
> DW_TAG_label debugging information entries without an address
attribute
> (DW_AT_low_pc, DW_AT_high_pc, DW_AT_ranges, or DW_AT_entry_pc) are
excluded.

The effect of this on the size of accelerator tables is minimal, as
TAG_labels are usually created by C/C++ labels (see example in test),
which are typically paired with "goto" statements.