fd656a0fbb
See [the documentation](https://cmake.org/cmake/help/v3.0/command/find_package.html)
for exactly how the search procedure works. If you want to use an
LLVM from a specific location, use CMAKE_PREFIX_PATH as normal.
No longer do we have a nonstandard HIPIFY_CLANG_LLVM_DIR variable
for people to learn about.
[ROCm/hip commit: 8fefc6a2b7]