[LIT] Fix testing out-of-tree Clang builds

[EricWF]

Summary

Currently, LIT configures the LLVM binary path before the Clang binary path. However this breaks testing out-of-tree Clang builds (where the LLVM binary path includes a copy of Clang).

This patch reverses the order of the paths when looking for Clang, putting the Clang binary directory first.

[mgorny]

To be honest, as I said before, I'm entirely confused by the logic there, with all the prepending, appending and shuffling around. But if you believe it gives the correct result, I'm all for it.

However, if that wouldn't be too much of a hassle, would you mind also renaming the variables to use a bit more meaningful names?

My alternative idea would be to put both paths on the initial list, and filter out None instances from the list afterwards, e.g. something like:

paths = [getattr(self.config, 'clang_tools_dir', None), self.config.llvm_tools_dir]
paths = [x for x in paths if x is not None]

[mgorny]

Or even more abstract (if we assume we don't have to check for presence of llvm_tools_dir):

path_vars = ['clang_tools_dir', 'llvm_tools_dir']
paths = [getattr(self.config, k) for k in path_vars if hasattr(self.config, k)]

(untested)

[EricWF]

• Use style suggested by @mgorny.

[EricWF]

Accepting for post-commit review. I don't want to be carrying this patch locally.

utils/lit/lit/llvm/config.py

def use_clang(self, required=True):
                                        'LIBCLANG_CODE_COMPLETION_LOGGING']
         # Clang/Win32 may refer to %INCLUDE%. vsvarsall.bat sets it.
         if platform.system() != 'Windows':
             possibly_dangerous_env_vars.append('INCLUDE')
 
         self.clear_environment(possibly_dangerous_env_vars)
 
         # Tweak the PATH to include the tools dir and the scripts dir.
-        paths = [self.config.llvm_tools_dir]
-        tools = getattr(self.config, 'clang_tools_dir', None)
-        if tools:
-            paths = paths + [tools]
+        # Put Clang first to avoid LLVM from overriding out-of-tree clang builds.
+        possible_paths = ['clang_tools_dir', 'llvm_tools_dir']
+        paths = [getattr(self.config, pp) for pp in possible_paths
+                 if getattr(self.config, pp, None)]
         self.with_environment('PATH', paths, append_path=True)
 
         paths = [self.config.llvm_shlib_dir, self.config.llvm_libs_dir]
         self.with_environment('LD_LIBRARY_PATH', paths, append_path=True)
 
         # Discover the 'clang' and 'clangcc' to use.
 
         self.config.clang = self.use_llvm_tool(