Implement digamma for XLA

Compute the Lgamma function using Lanczos' approximation from "A Precision Approximation of the Gamma Function". SIAM Journal on Numerical Analysis series B. Vol. 1:
digamma(z + 1) = log(t(z)) + A'(z) / A(z) - kLanczosGamma / t(z)
t(z) = z + kLanczosGamma + 1/2
A(z) = kBaseLanczosCoeff + sigma(k = 1, n, kLanczosCoefficients[i] / (z + k))
A'(z) = sigma(k = 1, n, kLanczosCoefficients[i] / (z + k) / (z + k))

PiperOrigin-RevId: 204834091