libcpp: Provide date routine

Joseph pointed me at cb_get_source_date_epoch, which allows repeatable
builds and solves a FIXME I had on the modules branch.  Unfortunately
it's used exclusively to generate __DATE__ and __TIME__ values, which
fallback to using a time(2) call.  It'd be nicer if the preprocessor
made whatever time value it determined available to the rest of the
compiler.  So this patch adds a new cpp_get_date function, which
abstracts the call to the get_source_date_epoch hook, or uses time
directly.  The value is cached.  Thus the timestamp I end up putting
on CMI files matches __DATE__ and __TIME__ expansions.  That seems
worthwhile.

	libcpp/
	* include/cpplib.h (enum class CPP_time_kind): New.
	(cpp_get_date): Declare.
	* internal.h (struct cpp_reader): Replace source_date_epoch with
	time_stamp and time_stamp_kind.
	* init.c (cpp_create_reader): Initialize them.
	* macro.c (_cpp_builtin_macro_text): Use cpp_get_date.
	(cpp_get_date): Broken out from _cpp_builtin_macro_text and
	genericized.

libcpp/internal.h

@@ -512,10 +512,9 @@ struct cpp_reader
   const unsigned char *date;
   const unsigned char *time;
 
-  /* Externally set timestamp to replace current date and time useful for
-     reproducibility.  It should be initialized to -2 (not yet set) and
-     set to -1 to disable it or to a non-negative value to enable it.  */
-  time_t source_date_epoch;
+  /* Time stamp, set idempotently lazily.  */
+  time_t time_stamp;
+  int time_stamp_kind; /* Or errno.  */
 
   /* A token forcing paste avoidance, and one demarking macro arguments.  */
   cpp_token avoid_paste;