The character type comment in exp_dbug.ads documented the QU and QW
encodings, but did not document the plain "Qc" encoding, where 'c' is
the character itself.  This patch updates the comment to follow the
implementation.

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

2019-09-17  Tom Tromey  <tromey@adacore.com>

gcc/ada/

	* exp_dbug.ads: Update character type comment.