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.