Add cplus_demangle_v3 wrapper to Demangle.cpp

Summary: This file contains a wrapper that is protected from `basename` fn name collision between `libiberty.h` and `libgen.h` (0fb6287719abb3cf52d7eda457efbe567985d1d4). I ran into this same issue again when trying to use `libiberty`'s `cplus_demangle_v3`, so let's add a similar wrapper for this function.

Reviewed By: fjricci

Differential Revision: D24008824

fbshipit-source-id: 868e90579f753f0c5b62aaeb27af2ef0a2006155

folly/detail/Demangle.cpp

@@ -48,5 +48,15 @@ int cplus_demangle_v3_callback_wrapper(
 #endif
 }
 
+char* cplus_demangle_v3_wrapper(const char* mangled) {
+#if FOLLY_DETAIL_HAVE_DEMANGLE_H
+  auto const options = DMGL_PARAMS | DMGL_ANSI | DMGL_TYPES;
+  return cplus_demangle_v3(mangled, options);
+#else
+  (void)mangled;
+  return 0;
+#endif
+}
+
 } // namespace detail
 } // namespace folly

folly/detail/Demangle.h

@@ -32,5 +32,7 @@ extern int cplus_demangle_v3_callback_wrapper(
     void (*cbref)(char const*, std::size_t, void*),
     void* opaque);
 
+extern char* cplus_demangle_v3_wrapper(const char* mangled);
+
 } // namespace detail
 } // namespace folly