Correct warning on Float->Integer conversions.

As identified and briefly discussed here:
https://bugs.llvm.org/show_bug.cgi?id=37305

Converting a floating point number to an integer type when
the integral part is out of the range of the integer type is
undefined behavior in C. Additionally, CodeGen emits an undef
in this situation.

HOWEVER, we've been giving a warning that says that the value is
changed. This patch corrects the warning to list that it is actually
undefined behavior.

Differential Revision: https://reviews.llvm.org/D46535

llvm-svn: 331673