Setting up CMake to default to Debug when no build type is specified.

Summary: Turns out if you don't set CMAKE_BUILD_TYPE the default is an empty string. This results in some of the behaviors of debug builds, but not all of them. For example ENABLE_ASSERTIONS is false.

Reviewers: rnk

Reviewed By: rnk

Subscribers: chapuni, llvm-commits

Differential Revision: http://reviews.llvm.org/D7360

llvm-svn: 230041