[gdb/testsuite] Add gdb.dwarf2/clang-debug-names.c

[gdb/testsuite] Add gdb.dwarf2/clang-debug-names.c

Add test-case with .debug_names section using DW_FORM_ref4.

There's currently no support for .debug_names in the dwarf assembler, so we
use plain _emit rather than something more structured.

Consequently, we cannot use regular declare_labels-generated labels to refer
from .debug_names to .debug_info.  Instead, we use labels with a
predefined name, which we generate using _compute_label, and then define using
define_label.

This is the test-case for PR25952, so kfail the corresponding test.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-08  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/clang-debug-names.c: New test.
	* gdb.dwarf2/clang-debug-names.exp: New file.