[debug] Add -gdescribe-dies

[debug] Add -gdescribe-dies

This patch adds option -gdescribe-dies.  It sets the DW_AT_description
attribute of dies that do not get a DW_AT_name attribute, to make it easier
to figure out what the die is describing.

The option exports the names of artificial variables:
...
 DIE    0: DW_TAG_variable (0x7fa934dd54b0)
+  DW_AT_description: "D.1922"
   DW_AT_type: die -> 0 (0x7fa934dd0d70)
   DW_AT_artificial: 1

...
which can be traced back to gimple dumps:
...
  char a[0:D.1922] [value-expr: *a.0];
...

Furthermore, it adds names to external references:
...
 DIE    0: DW_TAG_subprogram (0x7fa88b9650f0)
+DW_AT_description: "main"
 DW_AT_abstract_origin: die -> label: vla_1.c.6719312a + 29 (0x7fa88b965140)
...
and likewise to DW_TAG_call_site_parameter DIEs.

Bootstrapped and reg-tested on x86_64.

2018-09-12  Tom de Vries  <tdevries@suse.de>

	* common.opt (gdescribe-dies): Add option.
	* dwarf2out.c (add_name_and_src_coords_attributes): Add description
	attribute for artifical and nameless decls.
	(dwarf2out_register_external_die): Add description attribute to
	external reference die.
	(add_desc_attribute): New functions.
	(gen_subprogram_die): Add description attribute to
	DW_TAG_call_site_parameter.
	* tree-pretty-print.c (print_generic_expr_to_str): New function.
	* tree-pretty-print.h (print_generic_expr_to_str): Declare.
	* doc/invoke.texi (@item Debugging Options): Add -gdescribe-dies and
	-gno-describe-dies.
	(@item -gdescribe-dies): Add.

From-SVN: r264229