[Ada] Spurious error and missing warning on static predicate

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch handles properly a static predicate on a scalar type that
is trivially true. Previous to this patch the compiler rejected the
predicate on the incorrect grounds that it was not a static expression.

Compiling bad_days.ads must yield:

   bad_days.ads:4:34: warning: predicate is redundant (always True)

---
package Bad_Days is
 type Day is (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday);
 subtype Day_Bad is Day with 
     Static_Predicate => Day_Bad in Day;
end Bad_Days;

Tested on x86_64-pc-linux-gnu, committed on trunk

2017-12-15  Ed Schonberg  <schonberg@adacore.com>

	* exp_ch4.adb (Expand_N_In): Do not replace a membership test on a
	scalar type with a validity test when the membership appears in a
	predicate expression, to prevent a spurious error when predicate is
	specified static.
	* sem_ch13.adb (Build_Predicate_Functions): Add warning if a static
	predicate, after constant-folding, reduces to True and is this
	redundant.
	* par-ch4.adb: Typo fixes and minor reformattings.

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