Convert DF_SCAN etc from #define to an enum

Whilst debugging an issue in df, I noticed that there are
some #define constants that could be an enum (thus making them known
to gdb).

Convert them to a new enum, and update the "id" field of
struct df_problem.

gcc/ChangeLog:
	* df.h (DF_SCAN, DF_LR, DF_LIVE, DF_RD, DF_CHAIN, DF_WORD_LR,
	DF_NOTE, DF_MD, DF_MIR, DF_LAST_PROBLEM_PLUS1): Convert from
	#define to...
	(enum df_problem_id): ...this new enum.
	(struct df_problem): Convert field "id" from "int" to
	enum df_problem_id.

From-SVN: r235522