[Fixed Point Arithmetic] Fixed Point Constant

This patch proposes an abstract type that represents fixed point numbers, similar to APInt or APSInt that was discussed in https://reviews.llvm.org/D48456#inline-425585. This type holds a value, scale, and saturation and is meant to perform intermediate calculations on constant fixed point values.

Currently this class is used as a way for handling the conversions between fixed point numbers with different sizes and radixes. For example, if I'm casting from a signed _Accum to a saturated unsigned short _Accum, I will need to check the value of the signed _Accum to see if it fits into the short _Accum which involves getting and comparing against the max/min values of the short _Accum. The FixedPointNumber class currently handles the radix shifting and extension when converting to a signed _Accum.

Differential Revision: https://reviews.llvm.org/D48661

llvm-svn: 339028