[PATCH] Don't warn about floating point comparison in unevaluated contexts

[Ondřej Majerech]

Testcase:

int
main() {
   double x = 1.0, y = 2.0;
   decltype(x == y) b{};
   (void) b;
}

With current Clang, the code above produces a warning:

/tmp/foo.cpp:4:14: warning: comparing floating point with == or != is unsafe
       [-Wfloat-equal]
   decltype(x == y) b{};

That warning is clearly useless as we are in an unevaluated context 
here. This patch suppresses this warning in these contexts.

There is no test for this patch, because I couldn't find any suitable 
test file to add it to. I'll be happy to add one if you can suggest a 
proper place for it.

~ Ondra
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