[CMake] Add custom target to create build directory

[CMake] Add custom target to create build directory

CMake does a poor job in tracking dependencies on files and directories
directly. Create custom target similar to the configuration step.
On my system, this avoids the reconfiguration on each build.

Differential Revision: https://reviews.llvm.org/D39298

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317694 91177308-0d34-0410-b5e6-96231b3b80d8