Use block_enum instead of int for better typesafety

Use block_enum instead of int for better typesafety

gdb/ChangeLog:

2019-07-21  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (lookup_symbol_in_objfile_symtabs): Change int to block_enum.
	(lookup_symbol_in_objfile): Change int to block_enum and add a
	gdb_assert to make sure block_index is GLOBAL_BLOCK or STATIC_BLOCK.