Modify clang detection to build clang (#261)

Per chandlerc's comment:

First, this builds inside the Bazel tree rather than in the source
repository. This ensures we start in a clean directory each time the
repository rule is run again. We don't need to detect an existing build
with this, and it will only be rebuilt when the workspace file or the
repository rule implementation is changed. This can be forced by using:
```
bazel sync --configure
```

Second, we use an implicit dependency on the `WORKSPACE` file to locate
the workspace directory automatically, and the `HEAD` file from the
`llvm-project` submodule to trigger a rebuild if the submodule is
updated.

Third, teach the CMake script to try to use a system-installed `clang`
if installed and not overridden by the `CC` environment variable. This
is very different from the prior logic -- this is only used with the
CMake build, and so should work with any system C++ compiler that can
build Clang and LLVM.

Lastly, this tweaks the CMake options to tune this build given that we
now fully control it and it will only be used in this context. This
still results in a 1.5gb build for me. =/ But its as small as I can make
it really. It's a frustrating long list, but I couldn't find a more
brief way of representing this.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

.bazelversion

@@ -1 +1 @@
-last_green
+latest