[lld] Use installed llvm_gtest in standalone builds

[mgorny]

Summary

Use the installed llvm_gtest library instead of rebuilding it locally
when standalone builds are used. This change is now required
as otherwise the build fails due to duplicate llvm_gtest target.
This is based on 82169103958583d3320b3a9a1e6542e8d32ef8da in clang.

[tstellar]

LGTM.

lld/CMakeLists.txt

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 ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       if(NOT LLVM_UTILS_PROVIDED)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/FileCheck utils/FileCheck)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/not utils/not)
         set(LLVM_UTILS_PROVIDED ON)
         set(LLD_TEST_DEPS FileCheck not)
       endif()
-      set(UNITTEST_DIR ${LLVM_THIRD_PARTY_DIR}/unittest)
-      if(EXISTS ${UNITTEST_DIR}/googletest/include/gtest/gtest.h
-          AND NOT EXISTS ${LLVM_LIBRARY_DIR}/${CMAKE_STATIC_LIBRARY_PREFIX}gtest${CMAKE_STATIC_LIBRARY_SUFFIX}
-          AND EXISTS ${UNITTEST_DIR}/CMakeLists.txt)
-        add_subdirectory(${UNITTEST_DIR} third-party/unittest)
+
+      if (NOT TARGET llvm_gtest)
+        message(FATAL_ERROR "llvm-gtest not found. Please install llvm-gtest or disable tests with -DLLVM_INCLUDE_TESTS=OFF")
       endif()
     else()
       # Seek installed Lit.
       find_program(LLVM_LIT
                    NAMES llvm-lit lit.py lit
                    PATHS "${LLVM_MAIN_SRC_DIR}/utils/lit"
                    DOC "Path to lit.py")
     endif()