[Constraint application] Stop optimizing casts in the AST.

Constraint application was rewriting conditional casts (as?) and
forced casts (as!) into coercions (as) at the AST level when it
determined that there was a coercion. Stop doing that: it's the
optimizer's job to remove such casts.

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -3583,9 +3583,6 @@ namespace {
         return nullptr;
       case CheckedCastKind::Coercion:
       case CheckedCastKind::BridgingCoercion: {
-        if (SuppressDiagnostics)
-          return nullptr;
-
         if (cs.getType(sub)->isEqual(toType)) {
           ctx.Diags.diagnose(expr->getLoc(), diag::forced_downcast_noop, toType)
               .fixItRemove(SourceRange(
@@ -3599,14 +3596,9 @@ namespace {
                             "as");
         }
 
-        // Transmute the checked cast into a coercion expression.
-        auto *result =
-            new (ctx) CoerceExpr(sub, expr->getLoc(), expr->getCastTypeLoc());
-        cs.setType(result, toType);
-        cs.setType(result->getCastTypeLoc(), toType);
-        unsigned disjunctionChoice =
-          (castKind == CheckedCastKind::Coercion ? 0 : 1);
-        return visitCoerceExpr(result, disjunctionChoice);
+        expr->setCastKind(castKind);
+        cs.setType(expr, toType);
+        return expr;
       }
 
       // Valid casts.
@@ -3659,37 +3651,18 @@ namespace {
           fromType, toType, castContextKind, cs.DC, expr->getLoc(), sub,
           expr->getCastTypeLoc().getSourceRange());
       switch (castKind) {
-        /// Invalid cast.
+      // Invalid cast.
       case CheckedCastKind::Unresolved:
         expr->setCastKind(CheckedCastKind::ValueCast);
         break;
 
       case CheckedCastKind::Coercion:
       case CheckedCastKind::BridgingCoercion: {
-        if (SuppressDiagnostics)
-          return nullptr;
-
         ctx.Diags.diagnose(expr->getLoc(), diag::conditional_downcast_coercion,
                            cs.getType(sub), toType);
-
-        // Transmute the checked cast into a coercion expression.
-        auto *coerce =
-            new (ctx) CoerceExpr(sub, expr->getLoc(), expr->getCastTypeLoc());
-        cs.setType(coerce, toType);
-        cs.setType(coerce->getCastTypeLoc(), toType);
-        unsigned disjunctionChoice =
-          (castKind == CheckedCastKind::Coercion ? 0 : 1);
-        Expr *result = visitCoerceExpr(coerce, disjunctionChoice);
-        if (!result)
-          return nullptr;
-
-        // Wrap the result in an optional. Mark the optional injection as
-        // explicit, because the user did in fact write the '?' as part of
-        // 'as?', even though it wasn't necessary.
-        result =
-            new (ctx) InjectIntoOptionalExpr(result, OptionalType::get(toType));
-        result->setImplicit(false);
-        return cs.cacheType(result);
+        expr->setCastKind(castKind);
+        cs.setType(expr, OptionalType::get(toType));
+        return expr;
       }
 
       // Valid casts.