Implement lgamma for XLA

Add support for Real and Imag for real floating point types.

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

PiperOrigin-RevId: 204815805