[Ada] Improve GNATprove messages on unproved checks

[Ada] Improve GNATprove messages on unproved checks

GNATprove messages may point out to part of an assertion as not being proved,
and in such a case it displays the sub-expression. This code relies on
Pprint.Expression_Image, which is improved here to display better some kinds of
expressions.

There is no impact on compilation.

2018-05-24  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* pprint.adb (Expression_Image): Improve the printing of expressions,
	by taking more cases into account, in particular qualified expressions
	and aggregates.  Also count more the number of parentheses to close
	after the expression.

From-SVN: r260665