[APFloat] Remove orphan ilogb(DoubleAPFloat) declaration. NFCI.

Author: RKSimon

llvm/include/llvm/ADT/APFloat.h

@@ -687,7 +687,6 @@ class DoubleAPFloat final : public APFloatBase {
 
   bool getExactInverse(APFloat *inv) const;
 
-  friend int ilogb(const DoubleAPFloat &Arg);
   friend DoubleAPFloat scalbn(const DoubleAPFloat &X, int Exp, roundingMode);
   friend DoubleAPFloat frexp(const DoubleAPFloat &X, int &Exp, roundingMode);
   friend hash_code hash_value(const DoubleAPFloat &Arg);