[helpers] Teach llvm-env to self-host

In addition to debug builds, this teaches llvm-env to create self-hosted
bots that will use the "build" directory of the branch to compile a self-hosted
version of the toolchain. It'll use clang and also lld, if available.

Review changes:

* Check for clang, not just build dir
* Use -x instead of -f
* Always export LLVM_SELFHOST
* Quote strings for good measure
* Using LLVM_CMAKE_FLAGS

Change-Id: I55af0c213ae06cb8b99a33d1a80a05f09fe64c85
2 files changed