Remove some variables in favor of using gdb::optional

Remove some variables in favor of using gdb::optional

While reading that code, I noticed that some variables essentially meant
whether to consider some other variable or not.  I think using
gdb::optional (which was not available when this code was written) is
clearer, as it embeds the used/not used predicate directly in the type
of the variable, making it harder to miss.

gdb/ChangeLog:

	* dwarf2read.c (struct dw2_symtab_iterator):
	<want_specific_block>: Remove.
	<block_index>: Change type to gdb::optional.
	(dw2_symtab_iter_init): Remove WANT_SPECIFIC_BLOCK parameter,
	change type of BLOCK_INDEX parameter to gdb::optional.
	(dw2_symtab_iter_next): Re-write in function of gdb::optional.
	(dw2_lookup_symbol): Don't pass argument for
	WANT_SPECIFIC_BLOCK.
	(dw2_expand_symtabs_for_function): Don't pass argument for
	WANT_SPECIFIC_BLOCK, pass empty optional for BLOCK_INDEX.
	(class dw2_debug_names_iterator)
	<dw2_debug_names_iterator>: Remove WANT_SPECIFIC_BLOCK
	parameter, change BLOCK_INDEX type to gdb::optional.
	<m_want_specific_block>: Remove.
	<m_block_index>: Change type to gdb::optional.
	(dw2_debug_names_iterator::next): Change type of IS_STATIC to
	gdb::optional.  Re-write in function of gdb::optional.
	(dw2_debug_names_lookup_symbol): Don't pass argument for
	WANT_SPECIFIC_BLOCK.
	(dw2_debug_names_expand_symtabs_for_function): Don't pass
	argument for WANT_SPECIFIC_BLOCK, pass empty optional for
	BLOCK_INDEX.