[PATCH] Refactor lit.cfg to use lit.util.which

[Robinson, Paul]

llvm/test/lit.cfg was doing tool-name substitutions using PATHEXT 
and os.path.exists().  Better to use lit.util.which, as a centralized
place that knows path-name quirks.
As a side effect, it's easy to tell if a tool doesn't exist, so I 
removed some non-existent tools from the list.  (Possibly 'clang' 
should also be removed? It shouldn't be used for any LLVM tests...)

Okay to submit?
Thanks,
--paulr

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