[Ada] Crash on inconsistent IF-expression

[Arnaud Charlet <charlet at adacore dot com>]

This change makes sure the compiler produces a proper error (rather
than crash) when compiling an (illegal) IF-expression where THEN-expression
is overloaded, and none of its interpretation is compatible with
the ELSE-expression.

The following compilation must display:

$ gcc -c badelse.adb
badelse.adb:4:50: type incompatible with that of "then" expression

package Badelse is
   type K is (Unknown, Blue, Red);
   type Tristate is (False, True, Unknown);
   Boo : Boolean;
   procedure P (X : K);
end Badelse;
package body Badelse is
   procedure P (X : K) is
   begin
      Boo := (if X = Unknown then Unknown else X = Blue);
   end P;
end Badelse;

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

2015-11-12  Thomas Quinot  <quinot@adacore.com>

	* sem_ch4.adb (analyze_If_Expression): Reject IF-expression where
	THEN-expression is overloaded and none of its interpretation is
	compatible with the ELSE-expression.

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