[Ada] Do not rewrite argument of pragma Entry_Queue_Length in analysis

[Ada] Do not rewrite argument of pragma Entry_Queue_Length in analysis

It was unusual for the analysis phase of compiler to rewrite static
expression of pragma Entry_Queue_Length with its value. This typically
happens when expanding the AST and only if needed for a given backend.
In particular, GNATprove doesn't need such an expansion and actually
needs the unrewritten AST to detect references to constants declared
with SPARK_Mode => Off within a code with SPARK_Mode => On.

This change has no impact on compilation, so no frontend test is
provided.

2019-08-21  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* sem_prag.adb (Max_Entry_Queue_Length): Do not substitute
	expression of the pragma argument with its value during
	analysis.
	* sem_util.adb (Get_Max_Queue_Length): Compute value of the
	pragma argument when needed.

From-SVN: r274781