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/gdbtypes.h

@@ -193,7 +193,7 @@ enum type_code
 /* * Some bits for the type's instance_flags word.  See the macros
    below for documentation on each bit.  */
 
-enum type_instance_flag_value
+enum type_instance_flag_value : unsigned
 {
   TYPE_INSTANCE_FLAG_CONST = (1 << 0),
   TYPE_INSTANCE_FLAG_VOLATILE = (1 << 1),