[CMake] Make omitting CMAKE_BUILD_TYPE an error

After a lot of discussion in this diff the consensus was that it is really hard to guess the users intention with their LLVM build. Instead of trying to guess if Debug or Release is the correct default option we opted for just not specifying CMAKE_BUILD_TYPE a error.

Discussion on discourse here:
https://discourse.llvm.org/t/rfc-select-a-better-linker-by-default-or-warn-about-using-bfd

Reviewed By: hans, mehdi_amini, aaron.ballman, jhenderson, MaskRay, awarzynski

Differential Revision: https://reviews.llvm.org/D124153