[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

CMakeLists.txt

@@ -51,7 +51,7 @@ project(LLVM
 
 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.