llvm.sh should default to https for BASE_URL vs. http

Currently the llvm.sh script up at https://apt.llvm.org/llvm.sh has the following:
```
BASE_URL="http://apt.llvm.org"
```

That BASE_URL is then used later for some wget validation. More and more networks are locking down the inbound and outbound use of http. A simple workaround is to just pass in "-m https://apt.llvm.org" to the script but it seems best if long term we just default to https in the script.