[Ada] Suppress warnings on membership test of ranges

[Ada] Suppress warnings on membership test of ranges

For a membership test "X in A .. B", the compiler used to warn if it
could prove that X is within one of the bounds.  For example, if we know
at compile time that X >= A, then the above could be replaced by "X <=
B".

This patch suppresses that warning, because there is really
nothing wrong with the membership test, and programmers sometimes
find it annoying.

gcc/ada/

	* exp_ch4.adb (Expand_N_In): Do not warn in the above-mentioned
	cases.
	* fe.h (Assume_No_Invalid_Values): Remove from fe.h, because
	this is not used in gigi.
	* opt.ads (Assume_No_Invalid_Values): Improve the comment. We
	don't need to "clearly prove"; we can just "prove". Remove the
	comment about fe.h, which is no longer true.