[CMake] Set default build type correctly

At least with cmake 3.6.1, the default build type setting was having no
effect; the generated CMakeCache.txt still had an empty CMAKE_BUILD_TYPE.
Force the variable to be set to achieve the desired behavior.

Differential Revision: https://reviews.llvm.org/D26200

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285789 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed