dwarf2out: reset generation count in toplev::finalize [PR98751]

dwarf2out: reset generation count in toplev::finalize [PR98751]

PR debug/98751 reports an issue in which most of libgccjit's tests
fails in DWARF 5 handling with
  `.Ldebug_loc2' is already defined"
asm errors.

The bogus label is being emitted at the 3rd in-process iteration, at:
  31673	      ASM_OUTPUT_LABEL (asm_out_file, loc_section_label);
which on the initial iteration emits:

 145   │ .Ldebug_loc0:

on the 2nd iteration:
 145   │ .Ldebug_loc1:

and on the 3rd iteration:
 145   │ .Ldebug_loc2:

which is a duplicate of a label emitted earlier:
 138   │     .section    .debug_loclists,"",@progbits
 139   │     .long   .Ldebug_loc3-.Ldebug_loc2
 140   │ .Ldebug_loc2:
 141   │     .value  0x5
 142   │     .byte   0x8
 143   │     .byte   0
 144   │     .long   0
 145   │ .Ldebug_loc2:

The issue seems to be that init_sections_and_labels creates the label
  ASM_GENERATE_INTERNAL_LABEL (loc_section_label, DEBUG_LOC_SECTION_LABEL,
			       generation);

where "generation" is a static local to init_sections_and_labels that
increments, and thus eventually hits the duplicate value.

It appears that this value is intended to be either 0 or 1, but in
the libgccjit case the compilation code can be invoked an arbitrary
number of times in-process, and hence can eventually lead to a
label name collision.

This patch adds code to dwarf2out_c_finalize (called by
toplev::finalize in libgccjit) to reset the generation counts,
fixing the issue.

gcc/ChangeLog:
	PR debug/98751
	* dwarf2out.c (output_line_info): Rename static variable
	"generation", moving it out of the function to...
	(output_line_info_generation): New.
	(init_sections_and_labels): Likewise, renaming the variable to...
	(init_sections_and_labels_generation): New.
	(dwarf2out_c_finalize): Reset the new variables.