[Flang] Wrong result for quad complex exponentiation to complex unit power

Flang produces incorrect results when exponentiating quad precision complex values to the complex unit power (1,0). The operation should return the input value unchanged, but instead returns incorrect results.

Expected Behavior: For any complex value z, the expression z ** (1,0) should return z unchanged. This is a mathematical identity: raising any number to the power of 1 equals the number itself.

Actual Behavior: When exponentiating certain quad precision complex values (such as (3,0)) to the complex unit power (1,0), the result differs from the input value by a small error.

Reproducer :

program test
  integer, parameter :: rk = selected_real_kind(30)

  real(rk), parameter :: x_ct = 3.0
  real(rk) :: x_rt
  if (real(x_ct**(1,0),rk) /= 3.0_rk) then
     print 2, real(x_ct**(1,0),rk)
  else
     print'("ok")'
  end if
  call s()
  if (real(x_rt**(1,0),rk) /= 3.0_rk) then
     print 2, real(x_rt**(1,0),rk)
  else
     print'("ok")'
  end if

2 Format ("Should be exactly 3.00...0", 1PE45.35)
contains
  subroutine s()
    character(3) :: src
    src="3.0"
    read (src,*) x_rt
  end subroutine s

end program test

Start by running the supplied Fortran reproducer with Flang and compare both exponentiation results against the expected exact value. Trace the implementation of quad-precision complex exponentiation for the complex unit power, then add or update coverage so the reproducer returns its input unchanged.