Use unsigned as base type for some enums

-fsanitize=undefined complains about using operator~ on various enum
types that are used with DEF_ENUM_FLAGS_TYPE.  This patch fixes these
problems by explicitly setting the base type for these enums to
unsigned.  It also adds a static assert to enum_flags to ensure that
future enums used this way have an unsigned underlying type.

gdb/ChangeLog
2018-10-03  Tom Tromey  <tom@tromey.com>

	* common/enum-flags.h (enum_flags::operator~): Add static assert.
	* symfile-add-flags.h (enum symfile_add_flag): Use unsigned as
	base type.
	* objfile-flags.h (enum objfile_flag): Use unsigned as base type.
	* gdbtypes.h (enum type_instance_flag_value): Use unsigned as base
	type.
	* c-lang.h (enum c_string_type_values): Use unsigned as base
	type.
	* btrace.h (enum btrace_thread_flag): Use unsigned as base type.

gdb/c-lang.h

@@ -35,7 +35,7 @@ struct parser_state;
 /* The various kinds of C string and character.  Note that these
    values are chosen so that they may be or'd together in certain
    ways.  */
-enum c_string_type_values
+enum c_string_type_values : unsigned
   {
     /* An ordinary string: "value".  */
     C_STRING = 0,