[Ada] Bug in composition of equality for variant records

[Ada] Bug in composition of equality for variant records

This patch fixes an omission in the construction of equality routines
for variant records, to take into account user-defined equality
functions for components of the record. Previously the constructed
equality routine for variant records used the predefined equality for
all components, When composavility of equality was introduced for
untagged records, expansion of record equality was modified properly,
but not for the case of variant records, which use a different and more
complex process to build the equality function.

2019-07-04  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch4.ads, exp_ch4.adb (Build_Eq_Call): New visible
	subprogram, extracted from Expand_Composite_Equality, to handle
	properly the composition of equality for variant record types.
	* exp_ch3.adb (MAke_Eq_If): Use Build_Eq_Call for each
	component, to handle properly the case of a component with a
	user-defined equality. Revert to predefined equality if the
	user-defined operation is abstract, to maintain compatibility
	with older versions,

gcc/testsuite/

	* gnat.dg/equal6.adb, gnat.dg/equal6_types.adb,
	gnat.dg/equal6_types.ads: New testcase.

From-SVN: r273062