[CMake][Standalone] Rewrite standalone build based on llvm-config. CLANG_PATH_TO_LLVM_*(s) are deprecated.

Checked on VS10(multiconfig) and some singleconfig builders.

* Assumptions

  - You should specify llvm-config as LLVM_CONFIG.
    CMake could find one in $PATH by default.

  - ENABLE_ASSERTIONS obeys LLVM's.

* Use cases

  a) With LLVM build tree

    Assume llvm-config is in your build tree.
    Everything should work as ever.

  b) With *installed* LLVM

    Assume distributions. The source tree can be optional.

    b1) The source tree is provided on the location `llvm-config --src-root`

      - Test utils, FileCheck &c., are imported and built in the new tree.
      - Gtest is built in the tree if gtest library is not found.
      - Lit is used in $(SRCROOT)/utils/lit/lit.py.

    b2) The source tree is not provided

      - clang and utilities can be built.
      - All tests, unittests and check-clang are invalidated and not built.