[CMake] Make LLVM_EXTERNAL_*_SOURCE_DIR consistent against older buildsites.

If corresponding in-tree subdirectory exists, just ignore LLVM_EXTERNAL* stuff.
Otherwise, set LLVM_TOOL_*_BUILD ON/OFF properly according to LLVM_EXTERNAL_*.

This makes easier to walk among old revisions *without* deleteing CMakeCache.txt.

Before r242059, LLVM_EXTERNAL_* was working like;

  if(EXISTS ${*_SOURCE_DIR}/CMakeLists.txt)
    set(*_BUILD ON CACHE)
    if(*_BUILD is ON)
      add_subdirectory(*_SOURCE_DIR)
    endif()
  endif()

llvm-svn: 245782