[TypeChecker] Don’t crash if a ExplicitCastExpr doesn’t have a cast type

rdar://95629905 discovered an issue where we are crashing because an `ExplicitCastExpr` doesn’t have a cast type. It looks like this only occurs if we perform `performSyntacticDiagnosticsForTarget` from inside  `applyResultBuilderBodyTransform`.

rdar://95629905

Author: ahoppen

lib/Sema/TypeCheckAvailability.cpp

@@ -3229,7 +3229,7 @@ class ExprAvailabilityWalker : public ASTWalker {
       maybeDiagParameterizedExistentialErasure(EE, Where);
     }
     if (auto *CC = dyn_cast<ExplicitCastExpr>(E)) {
-      if (!isa<CoerceExpr>(CC) &&
+      if (!isa<CoerceExpr>(CC) && CC->getCastType() &&
           CC->getCastType()->hasParameterizedExistential()) {
         SourceLoc loc = CC->getCastTypeRepr() ? CC->getCastTypeRepr()->getLoc()
                                               : E->getLoc();