[PATCH] Allow external projects to run tests via llvm-lit

[Greg Fitzgerald]

This file "cmake/modules/AddLLVM.cmake" ends up in the LLVM install directory.  But from that location, the user cannot use the lit macros unless they set LLVM_MAIN_SRC_DIR.  Reaching into the source directory is not necessary because the freshly built 'llvm-lit' is also in the LLVM install directory.  This patch allows the build to find llvm-lit.

http://reviews.llvm.org/D3429