The 'X' printf type has a valid alternative form. Fixes PR8641.

llvm-svn: 119946
diff --git a/clang/test/Sema/format-strings.c b/clang/test/Sema/format-strings.c
index 57f087b..ba5fa8a 100644
--- a/clang/test/Sema/format-strings.c
+++ b/clang/test/Sema/format-strings.c
@@ -325,3 +325,8 @@
   rdar8332221_vprintf_scanf("%", ap, "%d", x); // expected-warning{{incomplete format specifier}}
 }
 
+// PR8641
+void pr8641() {
+  printf("%#x\n", 10);
+  printf("%#X\n", 10);
+}