[LLD][CMake] fix testing standalone build without installed llvm-lit. (#138575)

This block was copied from clang/CMakeLists.txt.

Author: jeremyd2019

lld/CMakeLists.txt

@@ -63,6 +63,9 @@ if(LLD_BUILT_STANDALONE)
     if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       # Note: path not really used, except for checking if lit was found
       set(LLVM_EXTERNAL_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/not utils/not)