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

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