cmake: Error on invalid CMAKE_BUILD_TYPE

cmake: Error on invalid CMAKE_BUILD_TYPE

Apparently if you make a typo in the argument to CMAKE_BUILD_TYPE,
cmake silently accepts this but doesn't apply any particular build
type to your build. This means you get a build that doesn't really
make any sense - it's sort of a debug build with asserts disabled.

Error out instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244406 91177308-0d34-0410-b5e6-96231b3b80d8