Add CodeView enum cv_sym_type

Add CodeView enum cv_sym_type

Make everything more gdb-friendly by using an enum for symbol constants
rather than #defines.

gcc/
	* dwarf2codeview.cc (S_LDATA32, S_GDATA32, S_COMPILE3): Undefine.
	(enum cv_sym_type): Define.
	(struct codeview_symbol): Use enum cv_sym_type.
	(write_codeview_symbols): Add default to switch.