Remove XFAIL in implicit_deduction_guides tests

The clang assertion causing these tests failing with sanitizer is fixed
in r295794. All the bots running libcxx tests should be upgraded and
running the compiler with the fix.

llvm-svn: 296385

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

@@ -10,10 +10,6 @@
 // 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