[Ada] Fix Constraint error on rexgexp close bracket find algorithm

[Pierre-Marie de Rodat]

In pattern syntax checking, make a procedure out of the algorithm to
find the close bracket matching an open bracket. Fix cases where the
close bracket is missing in the special cases '-' and '\', e.g.:

- "[a-b"
- "[\b"
- "[\]" misses either a backslash or a close bracket

These three cases would raise constraint errors.

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

gcc/ada/

	* libgnat/s-regexp.adb (Check_Well_Formed_Pattern): Fix
	Constraint_Error on missing close bracket.
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