[lldb] Skip declaration DIEs in the debug_names index (#94744)

This makes sure we try to process declaration DIEs that are erroneously
present in the index. Until bd5c6367, clang was emitting index
entries for declaration DIEs with DW_AT_signature attributes. This makes
sure to avoid returning those DIEs as the definitions of a type, but
also makes sure to pass through DIEs referring to static constexpr
member variables, which is a (probably nonconforming) extension used by
dsymutil.

It adds test cases for both of the scenarios. It is essentially a
recommit of #91808.