Add context to why test was disabled on Windows

test/Modules/odr_hash-Friend.cpp triggers an assertion in MicrosoftMangle.cpp
This has been reported in PR35939

llvm-svn: 322593
diff --git a/clang/test/Modules/odr_hash-Friend.cpp b/clang/test/Modules/odr_hash-Friend.cpp
index 5aafb31..39c0c4b 100644
--- a/clang/test/Modules/odr_hash-Friend.cpp
+++ b/clang/test/Modules/odr_hash-Friend.cpp
@@ -8,6 +8,7 @@
 // RUN:  -fmodules-cache-path=%t/modules.cache \
 // RUN:  -std=c++11 -x c++ %s -verify
 
+// PR35939: MicrosoftMangle.cpp triggers an assertion failure on this test.
 // UNSUPPORTED: system-windows
 
 // expected-no-diagnostics