[Ada] Set the flag In_Assertion_Expr during analysis of assertion expressions

[Arnaud Charlet <charlet at adacore dot com>]

In_Assertion_Expr should be non-zero during analysis of assertion expressions,
even for preanalysis of these expressions. So wrap the call to
Preanalyze_Spec_Expression to provide proper increment/decrement of the flag
for assertion expressions.

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

2012-10-01  Yannick Moy  <moy@adacore.com>

	* sem_ch13.adb (Add_Invariants): Analyze the invariant expression
	as an assertion expression.
	* sem_ch3.adb / sem_ch3.ads (Preanalyze_Assert_Expression):
	New procedure that wraps a call to Preanalyze_Spec_Expression
	for assertion expressions, so that In_Assertion_Expr can be
	properly adjusted.
	* sem_prag.adb (Analyze_PPC_In_Decl_Part
	Check_Precondition_Postcondition Preanalyze_CTC_Args): Call the
	new Preanalyze_Assert_Expression.

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