Convert c_string_type to an enum flags type

Convert c_string_type to an enum flags type

c_string_type contains values meant to be OR'ed together (even though
some bits are mutually exclusive), so it makes sense to make it an
enum flags type.

gdb/ChangeLog:
2015-11-17  Simon Marchi  <simon.marchi@ericsson.com>

	* c-exp.y (exp): Adjust, change enum c_string_type to
	c_string_type.
	(parse_string_or_char): Likewise.
	* c-lang.c (charset_for_string_type): Likewise.
	(classify_type): Likewise.
	(c_printchar): Likewise.
	(c_printstr): Likewise.
	(evaluate_subexp_c): Likewise.  And change cast to enum
	c_string_type_values.
	* c-lang.h: Include "common/enum_flags.h".
	(enum c_string_type): Rename to...
	(enum c_string_type_values): ...this.
	(c_string_type): Define new enum flags type.