fix ExceptionTracerTest build

Summary:
A recent diff added [[noreturn]] annotations that cause buck
build to fail for ExceptionTracerTest.cpp, due to our use of -Werror
and -Wunreachable-code-return.  This diff removes the unreachable return
statement.

Differential Revision: D4326888

fbshipit-source-id: 7863c163eedcf2a7e19730c1de194f05c544a2c0

folly/experimental/exception_tracer/ExceptionTracerTest.cpp

@@ -94,5 +94,5 @@ int main(int /* argc */, char* /* argv */ []) {
   testExceptionPtr1();
   testExceptionPtr2();
   baz();
-  return 0;
+  // no return because baz() is [[noreturn]]
 }