[cmake] Use llvm-lit directory when provided for stand-alone build

After the recent lit test changes, clang attempts to run its tests
via llvm-lit by default. However, the llvm-lit binary is not present
when performing stand-alone build resulting in a failure out of the box.

To solve that, add the llvm-lit directory to CMake when performing
a stand-alone build and LLVM sources are provided. This includes
the CMake rules generating the llvm-lit binary and effectively makes
it possible for clang to use it.

Differential Revision: https://reviews.llvm.org/D40142

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@318562 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -132,6 +132,9 @@ Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
     if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       # Note: path not really used, except for checking if lit was found
       set(LLVM_LIT ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
+      if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/llvm-lit)
+        add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/llvm-lit utils/llvm-lit)
+      endif()
       if(NOT LLVM_UTILS_PROVIDED)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/FileCheck utils/FileCheck)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/count utils/count)