Improve performance of solving over apply expressions by directly applying the return type whenever possible. This has some nice side-effects:

- Addresses many common user-reported "expression too complex" bugs, including rdar://problem/18876786.
- Shaves up to 10% off of the total time to run our unit tests. (Unscientifically measured on my iMac: 427.46s before, 385.17s after.)

Swift SVN r24514

lib/Sema/CSSimplify.cpp

@@ -3335,9 +3335,6 @@ retry:
   if (desugar2->getKind() == TypeKind::Function) {
     auto func2 = cast<FunctionType>(desugar2);
 
-    assert(func1->getResult()->is<TypeVariableType>() &&
-           "the output of funct1 is a free variable by construction");
-
     // If this application is part of an operator, then we allow an implicit
     // lvalue to be compatible with inout arguments.  This is used by
     // assignment operators.