Use cmake's builtin mechanism for handling library locations

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.

hipify-clang/CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 2.8.8)
 project(hipify-clang)
 
-find_package(LLVM PATHS ${HIPIFY_CLANG_LLVM_DIR} REQUIRED NO_DEFAULT_PATH)
+find_package(LLVM REQUIRED)
 message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
 
 option(HIPIFY_CLANG_TESTS "Build the tests for hipify-clang, if lit is installed" OFF)