[ADT] Move FixedPoint.h from Clang to LLVM.
This patch moves FixedPointSemantics and APFixedPoint
from Clang to LLVM ADT.
This will make it easier to use the fixed-point
classes in LLVM for constructing an IR builder for
fixed-point and for reusing the APFixedPoint class
for constant evaluation purposes.
Reviewed By: leonardchan, rjmccall
Differential Revision: https://reviews.llvm.org/D85312