[CMake] Generate ${BUILD_MODE}/llvm-lit for each ${CMAKE_CONFIGURATION_TYPES}.

llvm-lit can be invoked;

  $ Release/bin/llvm-lit

instead of;

  $ bin/llvm-lit --param buid_mode=Release

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198206 91177308-0d34-0410-b5e6-96231b3b80d8