This patch adds additional tags for warning messages.

Tested on x86_64-pc-linux-gnu, committed on trunk

2013-01-02  Robert Dewar  <dewar@adacore.com>

	* checks.adb, exp_ch4.adb, exp_ch6.adb, exp_ch7.adb, exp_ch9.adb,
	exp_disp.adb, exp_dist.adb, exp_intr.adb, exp_prag.adb, exp_util.adb,
	freeze.adb, gnat1drv.adb, inline.adb, layout.adb, lib-xref.adb,
	par-ch10.adb, par-labl.adb, par-load.adb, par-util.adb, restrict.adb,
	sem_ch13.adb, sem_ch4.adb, sem_ch6.adb, sem_dim.adb, sem_elab.adb,
	sem_res.adb, sem_warn.adb, sinput-l.adb: Add tags to warning messages.
	* sem_ch6.ads, warnsw.ads, opt.ads: Minor comment updates.