Sema: print type alias underlying types when debugging generics

With the recent change to StructuralTypeRequest where all returned types
are wrapped in a TypeAliasType, printing the requirement signature
could display requirements such as `Self.P3Assoc == Self.P3Assoc`.
This change will make sure that when in debug mode, the underlying type
will be displayed instead on the RHS.

Author: xymus

lib/Sema/TypeCheckDecl.cpp

@@ -2930,7 +2930,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       PD->dumpRef(llvm::errs());
       llvm::errs() << "\n";
       llvm::errs() << "Requirement signature: ";
-      requirementsSig->print(llvm::errs());
+
+      PrintOptions opts;
+      opts.PrintTypeAliasUnderlyingType = true;
+      requirementsSig->print(llvm::errs(), opts);
       llvm::errs() << "\n";
 
       // Note: One cannot canonicalize a requirement signature, because