Add an empty constraint propagation pass.

This is disabled by default but enabled under the frontend option
-propagate-constraints.

The idea here is to have a pass that enforces local consistency in our
constraint system, in order to reduce the domains of constraint
variables, speeding up the solving of the constraint system.

The initial focus will be on reducing the size of the disjunctions for
function overloads with the hope that it substantially improves the
performance of type checking many expressions.

lib/Frontend/CompilerInvocation.cpp

@@ -864,6 +864,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   
   Opts.EnableASTScopeLookup |= Args.hasArg(OPT_enable_astscope_lookup);
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
+  Opts.EnableConstraintPropagation |= Args.hasArg(OPT_propagate_constraints);
   Opts.IterativeTypeChecker |= Args.hasArg(OPT_iterative_type_checker);
   Opts.DebugGenericSignatures |= Args.hasArg(OPT_debug_generic_signatures);