Track from which abstract lexical block concrete ones come from in DWARF
so that debuggers can inherit the former from the latter. This enables
debuggers to properly handle the following case:

  * function Child2 is nested in a lexical block, itself nested in
    function Child1;
  * function Child1 is inlined into some call site;
  * function Child2 is never inlined.

Here, Child2 is described in DWARF only in the abstract instance of
Child1. So when debuggers decode Child1's concrete instances, they need
to fetch the definition for Child2 in the corresponding abstract
instance: the DW_AT_abstract_origin link on the lexical block that
embeds Child1 enables them to do that.

Bootstrapped and regtested on x86_64-linux.

gcc/ChangeLog:

	* dwarf2out.c (add_abstract_origin_attribute): Adjust
	documentation comment.  For BLOCK nodes, add a
	DW_AT_abstract_origin attribute that points to the DIE generated
	for the origin BLOCK.
	(gen_lexical_block_die): Call add_abstract_origin_attribute for
	blocks from inlined functions.

gcc/testsuite/Changelog:

	* gcc.dg/debug/dwarf2/nested_fun.c: New testcase.

From-SVN: r236065