[CMake] Enhance add_llvm_external_project.

  - Substitute hyphen to underscore, s/-/_/g, as the variable name.
  - Additional parameter can be specified as the name of directory.

e.g.) add_llvm_external_project(clang-tools-extra extra)

  - LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR=/path/to/llvm-srcroot/tools/clang/tools/extra, by default.
  - Build directory is in ${CMAKE_CURRENT_BINARY_DIR}/extra

llvm-svn: 165311