Building debug via CMake non standard.

It is common to configure a debug build with the syntax:
cmake -DCMAKE_BUILD_TYPE=Debug
In ROOT; however, that means build ROOT as Debug, but build LLVM/interpreter as Release.
Beginning at line 84 interpreter/CMakeLists.txt it starts to blindly override CMAKE_BUILD_TYPE based on the variable LLVM_BUILD_TYPE

Makes more sense to build everything the same via CMAKE_BUILD_TYPE, with an option to override how LLVM is built with LLVM_BUILD_TYPE.