[Ada] Replace low-level calls to Ekind with high-level calls to Is_Formal

[Ada] Replace low-level calls to Ekind with high-level calls to Is_Formal

High-level wrappers are easier to read. This change came up while reading
some code related to GNATprove, but then uniformly applied to the entire
frontend. For the few remaining membership tests that could be replaced
by Is_Formal it is not obvious whether the high-level routine makes the
code better.

2018-07-31  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* exp_aggr.adb, exp_ch4.adb, exp_ch6.adb, lib-xref.adb,
	repinfo.adb, sem_ch9.adb: Minor replace Ekind membership tests
	with a wrapper routine.

From-SVN: r263093