[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)},
                                                    ^~~~~~~~~~~~~~~~~~~~~~


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337968 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed