APFloat: Add some missing function declarations

Theses functions were added in fd1e5aa
without a declaration.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D154765

Author: tstellar

llvm/include/llvm/ADT/APFloat.h

@@ -770,6 +770,8 @@ class DoubleAPFloat final : public APFloatBase {
 };
 
 hash_code hash_value(const DoubleAPFloat &Arg);
+DoubleAPFloat scalbn(const DoubleAPFloat &Arg, int Exp, IEEEFloat::roundingMode RM);
+DoubleAPFloat frexp(const DoubleAPFloat &X, int &Exp, IEEEFloat::roundingMode);
 
 } // End detail namespace