Unhide _string_literal

[inaki-amatria]

The title is self-explanatory. I added a test case that covers all contexts where a prefixed string literal is valid in Fortran:

subroutine foo()
  integer, parameter :: k = 4
  character(len=128, kind=k) :: baz

  data baz / k_"baz" /

  open(unit=1, file=1_"mux")

  baz = k_"bux"
  pause 1_"fux"
end