[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.

RFC: http://lists.llvm.org/pipermail/llvm-dev/2020-August/144025.html

Reviewed By: leonardchan, rjmccall

Differential Revision: https://reviews.llvm.org/D85312