Make struct symbol inherit from general_symbol_info

Make struct symbol inherit from general_symbol_info

Since this is now no longer a POD, also give it a constructor that
initializes all fields. (I have considered overloading operator new
to zero-initialize the memory instead; let me know if you prefer that)

gdb/ChangeLog:

2019-11-12  Christian Biesinger  <cbiesinger@google.com>

	* ada-exp.y (write_ambiguous_var): Update.
	* buildsym.c (add_symbol_to_list): Update.
	* dwarf2read.c (read_variable): Update.
	(new_symbol): Update.
	* jit.c (finalize_symtab): Update.
	* language.c (language_alloc_type_symbol): Update.
	* symtab.c (fixup_symbol_section): Update.
	(initialize_objfile_symbol_1): Move code to...
	(initialize_objfile_symbol): ...here. Remove now-unnecessary memset.
	(allocate_symbol): Update.
	(allocate_template_symbol): Update.
	(get_symbol_address): Update.
	* symtab.h (struct symbol): Inherit from general_symbol_info instead
	of having as a field, and add a constructor.
	(SYMBOL_VALUE): Update.
	(SYMBOL_VALUE_ADDRESS): Update.
	(SET_SYMBOL_VALUE_ADDRESS): Update.
	(SYMBOL_VALUE_BYTES): Update.
	(SYMBOL_VALUE_COMMON_BLOCK): Update.
	(SYMBOL_BLOCK_VALUE): Update.
	(SYMBOL_VALUE_CHAIN): Update.
	(SYMBOL_LANGUAGE): Update.
	(SYMBOL_SECTION): Update.
	(SYMBOL_OBJ_SECTION): Update.
	(SYMBOL_SET_LANGUAGE): Update.
	(SYMBOL_SET_LINKAGE_NAME): Update.
	(SYMBOL_SET_NAMES): Update.
	(SYMBOL_NATURAL_NAME): Update.
	(SYMBOL_LINKAGE_NAME): Update.
	(SYMBOL_DEMANGLED_NAME): Update.
	(SYMBOL_SEARCH_NAME): Update.
	(SYMBOL_MATCHES_SEARCH_NAME): Update.
	(struct symbol): Update.
	(struct template_symbol): Update.
	(struct rust_vtable_symbol): Update.
	* xcoffread.c (SYMBOL_DUP): Update.

Change-Id: I05b1628455bcce3efaa101e65ef051708d17eb07