[APFloat] Add DoubleAPFloat mode to APFloat. NFC.

Summary:
This patch adds DoubleAPFloat mode to APFloat.

Now, an APFloat with semantics PPCDoubleDouble will have DoubleAPFloat layout
(APFloat.U.Double), which contains two underlying APFloats as
PPCDoubleDoubleImpl and IEEEdouble semantics. Currently the IEEEdouble APFloat
is not used, and the first APFloat behaves exactly the same before this change.

This patch consists of three kinds of logics:
1) Construction and destruction of APFloat. Now the ctors, dtor, assign
   opertors and factory functions construct different underlying layout
   based on the semantics passed in.
2) s/IEEE/getIEEE()/ for normal, lifetime-unrelated computation functions.
   These functions only access Floats[0] in DoubleAPFloat, which is the
   same as today's semantic.
3) A "Double dispatch" function, APFloat::convert. Converting between two
   different layouts requires appropriate logic.

Neither of these change the external behavior.

Reviewers: hfinkel, kbarton, echristo, iteratee

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D25977

llvm-svn: 285351