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.

gdb/cp-namespace.c

@@ -217,7 +217,7 @@ cp_lookup_bare_symbol (const struct language_defn *langdef,
 	lang_this = lookup_language_this (langdef, block);
 
       if (lang_this.symbol == NULL)
-	return null_block_symbol;
+	return {};
 
 
       type = check_typedef (TYPE_TARGET_TYPE (SYMBOL_TYPE (lang_this.symbol)));
@@ -225,7 +225,7 @@ cp_lookup_bare_symbol (const struct language_defn *langdef,
 	 This can happen for lambda functions compiled with clang++,
 	 which outputs no name for the container class.  */
       if (TYPE_NAME (type) == NULL)
-	return null_block_symbol;
+	return {};
 
       /* Look for symbol NAME in this class.  */
       sym = cp_lookup_nested_symbol (type, name, block, domain);
@@ -252,7 +252,7 @@ cp_search_static_and_baseclasses (const char *name,
 {
   /* Check for malformed input.  */
   if (prefix_len + 2 > strlen (name) || name[prefix_len + 1] != ':')
-    return null_block_symbol;
+    return {};
 
   /* The class, namespace or function name is everything up to and
      including PREFIX_LEN.  */
@@ -272,7 +272,7 @@ cp_search_static_and_baseclasses (const char *name,
   if (scope_sym.symbol == NULL)
     scope_sym = lookup_global_symbol (scope.c_str (), block, VAR_DOMAIN);
   if (scope_sym.symbol == NULL)
-    return null_block_symbol;
+    return {};
 
   struct type *scope_type = SYMBOL_TYPE (scope_sym.symbol);
 
@@ -379,13 +379,10 @@ cp_lookup_symbol_via_imports (const char *scope,
 			      const int search_parents)
 {
   struct using_direct *current;
-  struct block_symbol sym;
+  struct block_symbol sym = {};
   int len;
   int directive_match;
 
-  sym.symbol = NULL;
-  sym.block = NULL;
-
   /* First, try to find the symbol in the given namespace if requested.  */
   if (search_scope_first)
     sym = cp_lookup_symbol_in_namespace (scope, name,
@@ -476,7 +473,7 @@ cp_lookup_symbol_via_imports (const char *scope,
 	}
     }
 
-  return null_block_symbol;
+  return {};
 }
 
 /* Helper function that searches an array of symbols for one named NAME.  */
@@ -621,7 +618,7 @@ cp_lookup_symbol_via_all_imports (const char *scope, const char *name,
       block = BLOCK_SUPERBLOCK (block);
     }
 
-  return null_block_symbol;
+  return {};
 }
 
 /* Searches for NAME in the current namespace, and by applying
@@ -808,10 +805,7 @@ find_symbol_in_baseclass (struct type *parent_type, const char *name,
 			  int is_in_anonymous)
 {
   int i;
-  struct block_symbol sym;
-
-  sym.symbol = NULL;
-  sym.block = NULL;
+  struct block_symbol sym = {};
 
   for (i = 0; i < TYPE_N_BASECLASSES (parent_type); ++i)
     {
@@ -902,7 +896,7 @@ cp_lookup_nested_symbol_1 (struct type *container_type,
 	return sym;
     }
 
-  return null_block_symbol;
+  return {};
 }
 
 /* Look up a symbol named NESTED_NAME that is nested inside the C++
@@ -979,7 +973,7 @@ cp_lookup_nested_symbol (struct type *parent_type,
 			      "cp_lookup_nested_symbol (...) = NULL"
 			      " (func/method)\n");
 	}
-      return null_block_symbol;
+      return {};
 
     default:
       internal_error (__FILE__, __LINE__,