[Ada] Warn for failure to parenthesize unary minus

[Arnaud Charlet <charlet at adacore dot com>]

This patch gives a warning when unary minus is used in a context that
requires parentheses, and no parentheses are present, as shown by the
following test:

     1. function Test_Mod_B (N: in Integer) return Integer is
     2. begin
     3.    return N mod -10;
                        |
        >>> parentheses required for unary minus

     4. end Test_Mod_B;

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

2012-10-29  Robert Dewar  <dewar@adacore.com>

	* par-ch4.adb (P_Primary): Warn on bad use of unary minus.

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