[AutoDiff] Allow tgmath differentiation of `Double` (#40991)
This allows automatic differentiation of concrete calls to math functions operating on `Double`. Such calls were not differentiable because the concrete math functions were imported from C, not the generic ones defined in Swift. The fix is to define derivatives for these C functions. `test/AutoDiff/stdlib/tgmath_derivatives.swift.gyb` was modified to test the new functions.
