generated clang toolchain tarballs should use the full version string

```
$ ~/git/popsicle/target/release/popsicle `pwd`/src/third_party/llvm-build/Release+Asserts/bin/clang
$HOME/.cache/popsicle/clang/clang-6.0.0.tar.gz
```

```
$ `pwd`/src/third_party/llvm-build/Release+Asserts/bin/clang --version
clang version 6.0.0 (trunk 313786)
```