[Constraint solver] Apply solution to the system before builder transform.

This makes sure we can find information in the constraint system during
application. Fixes rdar://problem/59239224

Author: DougGregor

lib/Sema/BuilderTransform.cpp

@@ -1101,6 +1101,9 @@ Optional<BraceStmt *> TypeChecker::applyFunctionBuilderBodyTransform(
     // The system was salvaged; continue on as if nothing happened.
   }
 
+  // FIXME: Shouldn't need to do this.
+  cs.applySolution(solutions.front());
+
   // Apply the solution to the function body.
   if (auto result = cs.applySolution(
           solutions.front(),

test/Constraints/function_builder_diags.swift

@@ -327,3 +327,17 @@ func checkImplicitSelfInClosure() {
     }
   }
 }
+
+// rdar://problem/59239224 - crash because some nodes don't have type
+// information during solution application.
+struct X<T> {
+  init(_: T) { }
+}
+
+@TupleBuilder func foo(cond: Bool) -> some Any {
+  if cond {
+    tuplify(cond) { x in
+      X(x)
+    }
+  }
+}