[HIPIFY] cmake changes
[HIPIFY] cmake changes

+ standalone build support
+ WIN MSVC support
+ disable lit testing by default
+ to enable lit testing -DHIPIFY_CLANG_TESTS=1 should be specified

Tested builds:
+ hipify-clang standalone           {Ubuntu 16.04|Windows 10} {Debug|Release} {x86|x64a} {g++|clang++|MSVC 2017} {llvm 3.8.0|llvm 3.9.0} {-DHIPIFY_CLANG_TESTS=1| }
+ HIP {-DHIPIFY_CLANG_LLVM_DIR=..|} {Ubuntu 16.04}            {Debug|Release} {x86|x64a} {hcc}                   {llvm 3.8.0|llvm 3.9.0} {-DHIPIFY_CLANG_TESTS=1| }

ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP