[Ada] Performance degradation with references

[Ada] Performance degradation with references

This patch modifies the creation of markers for variable references in the
context of SPARK elaboration checks. Previously, prior to checking whether a
reference requires such a marker, the compiler performed a logarithmic look up
to determine whether the reference appears within a call. This action caused
the compiler to degrade when a source program contains multiple (100,000s)
references. Now, the compiler no longer performs the look up immediately.

2018-06-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_ch8.adb (Find_Direct_Name): Mode the declaration of
	Is_Assignment_LHS further in. Use predicate
	Needs_Variable_Reference_Marker to determine whether to create a
	variable marker.
	(Find_Expanded_Name): Mode the declaration of Is_Assignment_LHS further
	in.  Use predicate Needs_Variable_Reference_Marker to determine whether
	to create a variable marker.
	* sem_elab.adb (Build_Variable_Reference_Marker): Remove the various
	checks that determine whether the identifier or expanded name is a
	suitable variable reference.  The checks are now performed by
	Needs_Variable_Reference_Marker.
	* sem_res.adb (Resolve_Actuals): Use predicate
	Needs_Variable_Reference_Marker to determine whether to create a
	variable marker.
	* sem_util.adb (Needs_Variable_Reference_Marker): New routine.
	* sem_util.ads (Needs_Variable_Reference_Marker): New routine.

From-SVN: r261400