compiler: remove "DIR/../" when joining relative import path

Otherwise if DIR does not exist, the path does not work. This matches
    what the gc cmd/compile tool does, because it calls path.Join.
    
    The test for this is the cmd/go tests, to be added in a follow-up CL.
    
    Reviewed-on: https://go-review.googlesource.com/45691

From-SVN: r249194

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-be5fa26b2b1b5d0755bc1c7ce25f3aa26bea9d9c
+c0840d5826abb713487b2d8a04ab249764b21010
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/import.cc

@@ -82,6 +82,25 @@ Import::open_package(const std::string& filename, Location location,
 	  // A special case.
 	  fn = relative_import_path;
 	}
+      else if (fn[0] == '.' && fn[1] == '.'
+	       && (fn[2] == '\0' || IS_DIR_SEPARATOR(fn[2])))
+	{
+	  // We are going to join relative_import_path and fn, and it
+	  // will look like DIR/../PATH.  But DIR does not necessarily
+	  // exist in this case, and if it doesn't the use of .. will
+	  // fail although it shouldn't.  The gc compiler uses
+	  // path.Join here, which cleans up the .., so we need to do
+	  // the same.
+	  size_t index;
+	  for (index = relative_import_path.length() - 1;
+	       index > 0 && !IS_DIR_SEPARATOR(relative_import_path[index]);
+	       index--)
+	    ;
+	  if (index > 0)
+	    fn = relative_import_path.substr(0, index) + fn.substr(2);
+	  else
+	    fn = relative_import_path + '/' + fn;
+	}
       else
 	fn = relative_import_path + '/' + fn;
       is_local = false;