compiler: Improve location info for function parameters.

Previously, errors reported for function parameters would point to the
    beginning of the function declaration instead of to the column for the
    given parameter.
    
    Reviewed-on: https://go-review.googlesource.com/11580

From-SVN: r225756

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-c8cb74e70fbe87b14bbd083730f52a68c79ec6bb
+83191e8e2cb9f47f7c1e6bcb9997f21163292612
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/gogo.cc

@@ -1694,7 +1694,7 @@ Gogo::start_function(const std::string& name, Function_type* type,
 	   ++p)
 	{
 	  Variable* param = new Variable(p->type(), NULL, false, true, false,
-					 location);
+					 p->location());
 	  if (is_varargs && p + 1 == parameters->end())
 	    param->set_is_varargs_parameter();