Unit test name-component bounds searching directly

Unit test name-component bounds searching directly

This commit factors out the name-components-vector building and bounds
searching out of dw2_expand_symtabs_matching_symbol into separate
functions, and adds unit tests that:

 - expose both the latent bug mentioned in the previous commit, and
   also,

 - for completeness exercise the 0xff character handling fixed in the
   previous commit more directly.

The actual fix for the now-exposed bug is left for the following
patch.

gdb/ChangeLog:
2017-11-21  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (mapped_index::name_components_casing): New field.
	(mapped_index) <build_name_components,
	find_name_components_bounds): Declare new methods.
	(mapped_index::find_name_components_bounds)
	(mapped_index::build_name_components): New methods, factored out
	from dw2_expand_symtabs_matching_symbol.
	(check_find_bounds_finds)
	(test_mapped_index_find_name_component_bounds): New.
	(run_test): Rename to ...
	(test_dw2_expand_symtabs_matching_symbol): ... this.
	(run_test): Reimplement.