compiler: add Type::message_name

As we move toward generics, the error messages need to be able
to refer to types in a readable manner.  Add that capability,
and use it today in AST dumps.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/536716

Author: ianlancetaylor

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-8c056e335cecec67d1d223a329b7ba4dac778a65
+1cb83a415e86ab4de0d436d277377d8fc060cb61
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/ast-dump.cc

@@ -223,14 +223,7 @@ Ast_dump_context::dump_type(const Type* t)
   if (t == NULL)
     this->ostream() << "(nil type)";
   else
-    // FIXME: write a type pretty printer instead of
-    // using mangled names.
-    if (this->gogo_ != NULL)
-      {
-	Backend_name bname;
-	t->backend_name(this->gogo_, &bname);
-	this->ostream() << "(" << bname.name() << ")";
-      }
+    this->ostream() << "(" << t->message_name() << ")";
 }
 
 // Dump a textual representation of a block to the