compiler: Accept out of range integer -> unicode conversions.

When converting a signed or unsigned integer value into a constant
    string, if the integer does not fit into the Go "int" type, the string
    will become "\uFFFD."
    
    Fixes golang/go#11525.
    
    Reviewed-on: https://go-review.googlesource.com/13906

From-SVN: r227395

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-65672c16004c6d6d0247b6691881d282ffca89e3
+a63e173b20baa1a48470dd31a1fb1f2704b37011
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -3039,6 +3039,25 @@ Type_conversion_expression::do_lower(Gogo*, Named_object*,
 	}
     }
 
+  // According to the language specification on string conversions
+  // (http://golang.org/ref/spec#Conversions_to_and_from_a_string_type):
+  // When converting an integer into a string, the string will be a UTF-8
+  // representation of the integer and integers "outside the range of valid
+  // Unicode code points are converted to '\uFFFD'."
+  if (type->is_string_type())
+    {
+      Numeric_constant nc;
+      if (val->numeric_constant_value(&nc) && nc.is_int())
+        {
+          // An integer value doesn't fit in the Unicode code point range if it
+          // overflows the Go "int" type or is negative.
+          unsigned long ul;
+          if (!nc.set_type(Type::lookup_integer_type("int"), false, location)
+              || nc.to_unsigned_long(&ul) == Numeric_constant::NC_UL_NEGATIVE)
+            return Expression::make_string("\ufffd", location);
+        }
+    }
+
   if (type->is_slice_type())
     {
       Type* element_type = type->array_type()->element_type()->forwarded();