[libc][NFC] Add supporting class for atof implementationThis change adds the High Precision Decimal described here:https://nigeltao.github.io/blog/2020/parse-number-f64-simple.htmlIt will be used
[libc][NFC] Add supporting class for atof implementationThis change adds the High Precision Decimal described here:https://nigeltao.github.io/blog/2020/parse-number-f64-simple.htmlIt will be used for the atof implementation later, but is complete andtested now.The code is inspired by the golang implmentation of the HPD class, whichcan be found here: https://github.com/golang/go/blob/release-branch.go1.16/src/strconv/decimal.goReviewed By: sivachandraDifferential Revision: https://reviews.llvm.org/D110454
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[libc] Add implementation of expm1f.Use expm1f(x) = exp(x) - 1 for |x| > ln(2).For |x| <= ln(2), divide it into 3 subintervals: [-ln2, -1/8], [-1/8, 1/8], [1/8, ln2]and use a degree-6 polynomial
[libc] Add implementation of expm1f.Use expm1f(x) = exp(x) - 1 for |x| > ln(2).For |x| <= ln(2), divide it into 3 subintervals: [-ln2, -1/8], [-1/8, 1/8], [1/8, ln2]and use a degree-6 polynomial approximation generated by Sollya's fpminmax for each interval.Errors < 1.5 ULPs when we use fma to evaluate the polynomials.Differential Revision: https://reviews.llvm.org/D101134