compiler: fix ICE in handling of rune constants.

This patch corrects an ICE in handling on constant expressions
such as

const r = ^'a'

Part of issue 9.

From-SVN: r187264

gcc/go/gofrontend/expressions.cc

@@ -3712,6 +3712,9 @@ Unary_expression::eval_constant(Operator op, const Numeric_constant* unc,
     return false;
 
   mpz_t uval;
+  if (unc->is_rune())
+    unc->get_rune(&uval);
+  else
     unc->get_int(&uval);
   mpz_t val;
   mpz_init(val);