[CMake] Set default build type correctly

[smeenai]

Summary

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.

[beanz]

Yep. LGTM.

llvm/trunk/CMakeLists.txt

 
 project(LLVM
   ${cmake_3_0_PROJ_VERSION}
   ${cmake_3_0_LANGUAGES}
   C CXX ASM)
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(STATUS "No build type selected, default to Debug")
-  set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)")
+  set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)
 endif()
 
 # This should only apply if you are both on an Apple host, and targeting Apple.
 if(CMAKE_HOST_APPLE AND APPLE)
   if(NOT CMAKE_XCRUN)
     find_program(CMAKE_XCRUN NAMES xcrun)
   endif()
   if(CMAKE_XCRUN)