Add a test case for the regression in -Wfor-loop-analysis caused by r273548.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273590 91177308-0d34-0410-b5e6-96231b3b80d8

test/SemaCXX/warn-loop-analysis.cpp

@@ -260,3 +260,9 @@ void test8() {
     i--;
   }
 }
+
+int f(int);
+void test9() {
+  // Don't warn when variable is defined by the loop condition.
+  for (int i = 0; int x = f(i); ++i) {}
+}