Remove null_block_symbol

Remove null_block_symbol

This removes null_block_symbol.  It seemed simpler to me to change
initializations and returns to use value initialization rather than
null_block_symbol.  This also fixes up a few spots where
initialization was done piecemeal.

gdb/ChangeLog
2019-03-24  Tom Tromey  <tom@tromey.com>

	* ada-lang.c (standard_lookup): Simplify initialization.
	(ada_lookup_symbol_nonlocal): Simplify return.
	* solib-spu.c (spu_lookup_lib_symbol): Simplify return.
	* solib-darwin.c (darwin_lookup_lib_symbol): Simplify return.
	* solib-svr4.c (elf_lookup_lib_symbol): Simplify return.
	* rust-lang.c (rust_lookup_symbol_nonlocal): Simplify
	initialization.
	* solib.c (solib_global_lookup): Simplify.
	* symtab.c (null_block_symbol): Remove.
	(symbol_cache_lookup): Simplify returns.
	(lookup_language_this): Simplify returns.
	(lookup_symbol_aux): Simplify return.
	(lookup_local_symbol): Simplify returns.
	(lookup_global_symbol_from_objfile): Simplify return.
	(lookup_symbol_in_objfile_symtabs)
	(lookup_symbol_in_objfile_from_linkage_name): Simplify return.
	(lookup_symbol_via_quick_fns, lookup_symbol_in_static_block)
	(lookup_static_symbol, lookup_global_symbol): Simplify return.
	* cp-namespace.c (cp_lookup_bare_symbol)
	(cp_search_static_and_baseclasses, cp_lookup_symbol_via_imports)
	(cp_lookup_symbol_via_all_imports, cp_lookup_nested_symbol_1)
	(cp_lookup_nested_symbol): Don't use null_block_symbol.
	(cp_lookup_symbol_via_imports): Simplify initialization.
	(find_symbol_in_baseclass): Likewise.
	* symtab.h (null_block_symbol): Remove.
	* d-namespace.c (d_lookup_symbol): Don't use null_block_symbol.
	(d_lookup_nested_symbol, d_lookup_symbol_imports)
	(d_lookup_symbol_module): Likewise.
	(find_symbol_in_baseclass): Simplify initialization.