The answer here is stale.
In newer versions of LLVM, there is an included tool to include LLVM inside a CMake project.
Please see the documentation of how to embed LLVM in your project

You'll thus have this in your project CMakeLists.txt:

cmake_minimum_required(VERSION 3.4.3)
project(SimpleProject)

find_package(LLVM REQUIRED CONFIG)

message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")

# Set your project compile flags.
# E.g. if using the C++ header files
# you will need to enable C++11 support
# for your compiler.

include_directories(${LLVM_INCLUDE_DIRS})
add_definitions(${LLVM_DEFINITIONS})

# Now build our tools
add_executable(simple-tool tool.cpp)

# Find the libraries that correspond to the LLVM components
# that we wish to use
llvm_map_components_to_libnames(llvm_libs support core irreader)

# Link against LLVM libraries
target_link_libraries(simple-tool ${llvm_libs})

This should work.
If you are having problems on windows, install the source package and not the binary package as suggested in this thread

if you have this error:

CommandLine Error: Option 'help-list' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options

replace the ${llvm_libs} in target_link_libraries() by LLVM you can see this github thread