Allow strings with printf/eval

Allow strings with printf/eval

PR 13098 explains that if a user attempts to use a string with either
`printf' (or `eval'), gdb returns an error (inferior not running):

(gdb) printf "%s\n", "hello"
evaluation of this expression requires the target program to be active

However, the parser can certainly handle this case:

(gdb) p "hello"
$1 = "hello"

This discrepancy occurs because printf_c_string does not handle
this specific case.  The passed-in value that we are attempting to print
as a string is TYPE_CODE_ARRAY but it's lval type is not_lval.

printf_c_string will only attempt to print a string from the value's
contents when !TYPE_CODE_PTR, lval is lval_internalvar, and the value's
type is considered a string type:

  if (value->type ()->code () != TYPE_CODE_PTR
      && value->lval () == lval_internalvar
      && c_is_string_type_p (value->type ()))
    {
      ...
    }

Otherwise, it attempts to read the value of the string from the target's
memory (which is what actually generates the "evaluation of this ..."
error message).