[libc] clean up FPUtil long doubles

Add quietNaNMask consts to FloatProperties and make LongDoubleBitsX86
clear the extra bits that aren't set when initializing with an 80 bit
long double.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D113625

GitOrigin-RevId: 18da0c0a287707b2f80880c14d77e4f071aae88d
2 files changed