Change lookup_symbol's block_index parameter type to block_enum

Change lookup_symbol's block_index parameter type to block_enum

The only two values valid to pass to the block_index parameter of
quick_symbol_functions::lookup_symbol are GLOBAL_BLOCK and STATIC_BLOCK,
part of enum block_enum.  Change the type of that parameter to
block_enum.

Change also the block_index field of dw2_symtab_iterator in the same
way..  This makes it consistent with dw2_debug_names_iterator, which
already uses block_enum for its block_index field.

This is a follow-up to this thread:

https://sourceware.org/ml/gdb-patches/2019-08/msg00097.html

gdb/ChangeLog:

	* dwarf2read.c (struct dw2_symtab_iterator) <block_index>:
	Change type to gdb::optional<block_enum>.
	(dw2_symtab_iter_init): Change block_index parameter type
	to gdb::optional<block_enum>.
	(dw2_lookup_symbol): Change block_index parameter
	type to block_enum.c
	(dw2_debug_names_lookup_symbol): Likewise.
	* psymtab.c (psym_lookup_symbol): Likewise.
	* symfile-debug.c (debug_qf_lookup_symbol): Likewise.
	* symfile.h (struct quick_symbol_functions) <lookup_symbol>:
	Likewise.