Fix a few ICEs and other problems with -fdebug-types-sections.

Fix a few ICEs and other problems with -fdebug-types-sections.

(1) If a function contains a local typedef of an anonymous structure, GCC
will generate a typedef DIE in the function, where the typedef DIE points
to a structure_type DIE at the top level.  That structure_type DIE, if
it's a non-POD, can contain ctor and dtor definitions.  This causes an
assertion in should_move_die_to_comdat to fail, as we have up to now
assumed that this could never happen.

(2) With --std=c++11, a template parameter can refer to a local type defined
within a function.  Because that local type doesn't qualify for its own
type unit, we copy it as an "unworthy" type into the type unit that refers
to it, but we copy too much, leading to a comdat type unit that contains a
DIE with subprogram definitions rather than declarations.  These DIEs may
have DW_AT_low_pc/high_pc or DW_AT_ranges attributes, and consequently can
refer to range list entries that don't get emitted because they're not
marked when the compile unit is scanned, eventually causing an undefined
symbol at link time.

(3) When a class template instantiation is moved into a separate type unit,
it can bring along a lot of other referenced types into the type unit,
especially if the template is derived from another (large) type that
does not have an actually have a type definition in a type unit of its
own. When there are many instantiations of the same template, we get
a lot of duplication, and in the worst case (a template with several
parameters, instantiated multiple times along each dimension), GCC
can end up taking a long time and exhausting available memory.

This combinatorial explosion is being caused by copy_decls_walk, where
it finds a type DIE that is referenced by the type unit, but is not
itself a type unit, and copies a declaration for that type into the
type unit in order to resolve the reference within the type unit.
In the process, copy_decls_walk also copies all of the children of
that DIE. In the case of a base class with member function templates,
every one of the instantiated member functions is copied into every
type unit that references the base class.

gcc/
        PR debug/60929
	* dwarf2out.c (should_move_die_to_comdat): A type definition
        can contain a subprogram definition, but don't move it to a
        comdat unit.
	(clone_as_declaration): Copy DW_AT_abstract_origin attribute.
        (generate_skeleton_bottom_up): Remove DW_AT_object_pointer attribute
        from original DIE.
	(clone_tree_hash): Rename to...
	(clone_tree_partial): ...this; change callers.  Copy
        DW_TAG_subprogram DIEs as declarations.
	(copy_decls_walk): Don't copy children of a declaration into a
        type unit.

gcc/testsuite/
        PR debug/60929
        * g++.dg/debug/dwarf2/dwarf4-nested.C: New test case.
        * g++.dg/debug/dwarf2/dwarf4-typedef.C: Add
        -fdebug-types-section flag.

From-SVN: r209812