[libc++] Follow-up to r337960: specify lambda's return type to avoid

-Wc++11-narrowing warning on Darwin

The internal CI produced the following diagnostic:
error: non-constant-expression cannot be narrowed from type 'long long' to '__darwin_suseconds_t' (aka 'int') in initializer list [-Wc++11-narrowing]
  struct ::timeval ConvertedTS[2] = {{TS[0].tv_sec, Convert(TS[0].tv_nsec)},
                                                    ^~~~~~~~~~~~~~~~~~~~~~

llvm-svn: 337968