Build & Test sections in readme are inacurate

[marcus-nilis]

Hi,

I'm getting started with this project and found that readme doesn't work well.

1st - building. It would help to add step of building llvm with -DLLVM_ENABLE_PROJECTS="clang;lld" after checking it out. I mean, inside ./llvm-project. Otherwise error #4 appears. Alternatively having llvm (clang + lld) in prerequsites may work good enough.

2nd - testing. The last test produces an error:

[303/304] Running the mlir-hlo regression tests
llvm-lit: .../mlir-hlo/llvm-project/llvm/utils/lit/lit/llvm/subst.py:122: note: Did not find mlir-cpu-runner in .../mlir-hlo/llvm-build:.../mlir-hlo/build/bin

The reason is that mlir_binary_dir doesn't actually points to the .../bin subdirrectory. Using llvm_tools_dir resolves the problem.
in ./tests/lit.cfg.py

 tool_dirs = [
-    config.mlir_binary_dir,
+    config.llvm_tools_dir,
     config.mlir_hlo_tools_dir,
 ]

or maybe add llvm_tools_dir to mlir_hlo_tools_dir in lit.site.cfg.py.in