[lit] Allow lit.which() to find executables which already have an extension

[zturner]

Summary

LLVM usually specifies executable names without extension so that
they work portably across platforms.  Occasionally, if you're writing
something platform specific, you might want to specify the extension.
For example, you might want to write a test that invokes a batch file.
It's awkward to say `foo` when the file is really called `foo.bat`, but
in this case `lit.util.which()` currently won't find it because it will
construct the filename `foo.bat.bat`.

[amccarth]

This looks fine to me. Is there a lit expert/owner who should also review?

[zturner]

In D84423#2170264, @amccarth wrote:

This looks fine to me. Is there a lit expert/owner who should also review?

Uhh, just me and Reid I think :)

[zturner]

Perhaps now you ;-)

[amccarth]

Well then, since Reid is out, this is good to go! :-)

llvm/utils/lit/lit/util.py

def which(command, paths=None):
 
     # Would be nice if Python had a lib function for this.
     if not paths:
         paths = os.defpath
 
     # Get suffixes to search.
     # On Cygwin, 'PATHEXT' may exist but it should not be used.
     if os.pathsep == ';':
-        pathext = os.environ.get('PATHEXT', '').split(';')
+        # Since this branch implies windows, it's safe to convert all extensions to lowercase.
+        assert(sys.platform == 'win32')
+        pathext = [x.lower() for x in os.environ.get('PATHEXT', '').split(';')]
+
+        # If the path was given to us with an extension already, ignore PATHEXT
+        _, current_ext = os.path.splitext(command)
+        if current_ext.lower() in pathext:
+            pathext = ['']
     else:
         pathext = ['']
 
     # Search the paths...
     for path in paths.split(os.pathsep):
+        p = os.path.join(path, command)
         for ext in pathext:
-            p = os.path.join(path, command + ext)
-            if os.path.exists(p) and not os.path.isdir(p):
-                return os.path.normcase(os.path.normpath(p))
+            p_with_ext = p + ext
+            if os.access(p_with_ext, os.X_OK):
+                return os.path.normcase(os.path.normpath(p_with_ext))
 
     return None
 
 
 def checkToolsPath(dir, tools):
     for tool in tools:
         if not os.path.exists(os.path.join(dir, tool)):
             return False