Added PR number for failing win64 tests.

llvm-svn: 177886
diff --git a/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp b/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp
index f4ffba7..6b632b0 100644
--- a/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp
+++ b/clang/test/Tooling/auto-detect-from-source-parent-of-cwd.cpp
@@ -8,4 +8,5 @@
 invalid;
 
 // REQUIRES: shell
+// PR15590
 // XFAIL: win64