To run the libbcc lit tests:
 * Ensure `llvm-rs-as` is built, either by doing a top-level `make
   checkbuild` or by doing `mm` from frameworks/compile/slang.
 * Ensure that LLVM and libbcc are built with
   `FORCE_BUILD_LLVM_COMPONENTS=true`.
 * Ensure `opt` is built from external/llvm, either by top-level `make
   checkbuild` or by doing `mm` from external/llvm.