Enhanced implementation of -Wfloat-equal to check for comparisons against

Enhanced implementation of -Wfloat-equal to check for comparisons against
floating-point literals that are represented exactly by the APFloat in
FloatingLiteral. For such literals, we do not emit a warning since such checks are
often performed in real code to see if a variable has changed from its original
value. This heuristic clearly can lead to false negatives, but the hope is it will
significantly reduce false positives to help make the compiler flag more useful.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44424 91177308-0d34-0410-b5e6-96231b3b80d8