Work around Clang assertion when testing C++17 deduction guides with '-g'.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@295417 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/basic.string/string.cons/implicit_deduction_guides.pass.cpp

@@ -10,6 +10,10 @@
 // UNSUPPORTED: c++98, c++03, c++11, c++14
 // UNSUPPORTED: libcpp-no-deduction-guides
 
+// FIXME(EricWF): As of 16/Feb/2017 Clang hits an assertion when compiling this
+// test with '-g' (as the sanitizers do).
+// XFAIL: ubsan, asan, msan, tsan
+
 // <string>
 
 // Test that the constructors offered by std::basic_string are formulated