We had a _LIBCPP_ASSERT commented out because gcc 4.9 didn't like it. We...

We had a _LIBCPP_ASSERT commented out because gcc 4.9 didn't like it. We (LLVM) now require GCC 5.1, so that's not a problem any more. Re-enable the assertion. Fixes PR#36863

llvm-svn: 362465