Perform component-wise substitution of pack expansions immediately.

Substitution of a pack expansion type may now produce a pack type.
We immediately expand that pack when transforming a tuple, a function
parameter, or a pack.

I had to duplicate the component-wise transformation logic in the
simplifyType transform, which I'm not pleased about, but a little
code duplication seemed a lot better than trying to unify the code
in two very different places.

I think we're very close to being able to assert that pack expansion
shapes are either pack archetypes or pack parameters; unfortunately,
the pack matchers intentionally produce expansions of packs, and I
didn't want to add that to an already-large patch.

lib/Sema/CSApply.cpp

@@ -7125,8 +7125,8 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
     auto *expansion = dyn_cast<PackExpansionExpr>(expr);
 
     auto *elementEnv = expansion->getGenericEnvironment();
-    auto toElementType = elementEnv->mapPackTypeIntoElementContext(
-        toExpansionType->getPatternType()->mapTypeOutOfContext());
+    auto toElementType = elementEnv->mapContextualPackTypeIntoElementContext(
+        toExpansionType->getPatternType());
 
     auto *pattern = coerceToType(expansion->getPatternExpr(),
                                  toElementType, locator);