Fix CMake error with stand-alone cling
Fix CMake error with stand-alone cling

Use absolute PATH when checking the existance of the textinput directory. This prevent this error:

CMake Error at cmake/modules/AddLLVM.cmake:312 (add_library):
  Cannot find source file:

    C:/build/workspace/cling-test/src/core/textinput/src/textinput/Editor.cpp

  Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp
  .hxx .in .txx
Call Stack (most recent call first):
  tools/cling/CMakeLists.txt:142 (llvm_add_library)
  tools/cling/lib/UserInterface/CMakeLists.txt:17 (add_cling_library)

karies/cling,root-mirror/cling,root-mirror/cling,perovic/cling,perovic/cling,marsupial/cling,marsupial/cling,root-mirror/cling,perovic/cling,marsupial/cling,root-mirror/cling,perovic/cling,root-mirror/cling,karies/cling,karies/cling,karies/cling,marsupial/cling,marsupial/cling,perovic/cling,karies/cling,marsupial/cling,karies/cling,root-mirror/cling