[cmake] Update lit search to match the one in LLVM

[cmake] Update lit search to match the one in LLVM

Update the lit search logic to support all names supported in LLVM
(since r283029). The search order (i.e. PATHS vs HINTS) does no really
matter since the established path is not used, except for determining
whether lit is available.

Differential Revision: https://reviews.llvm.org/D23745

llvm-svn: 284496