[ASTDumper] Don't try to print opaque type decl substitutions

Attempting to dump opaque decl while type-checking a body of
the declaration it's associated with results in a request
cycle because underlying substitutions request would trigger
type-checking if the body isn't type-checked yet.

Author: xedin

lib/AST/ASTDumper.cpp

@@ -1171,10 +1171,6 @@ namespace {
 
       }, "opaque_interface", TypeColor);
 
-      if (auto underlyingSubs = OTD->getUniqueUnderlyingTypeSubstitutions()) {
-        printRec(*underlyingSubs);
-      }
-
       printFoot();
     }