clang/cmake: Use installed gtest libraries for stand-alone builds

(re-commit of cb38df4 with the correct commit message)

Reviewed By: phosek, kwk

Differential Revision: https://reviews.llvm.org/D138472

Author: tstellar

clang/CMakeLists.txt

@@ -118,12 +118,10 @@ if(CLANG_BUILT_STANDALONE)
         set(LLVM_UTILS_PROVIDED ON)
         set(CLANG_TEST_DEPS FileCheck count 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)
-      endif()
+    endif()
+
+    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()
 
     if(LLVM_LIT)
@@ -506,13 +504,11 @@ endif()
 
 
 if( CLANG_INCLUDE_TESTS )
-  if(EXISTS ${LLVM_THIRD_PARTY_DIR}/unittest/googletest/include/gtest/gtest.h)
-    add_subdirectory(unittests)
-    list(APPEND CLANG_TEST_DEPS ClangUnitTests)
-    list(APPEND CLANG_TEST_PARAMS
-      clang_unit_site_config=${CMAKE_CURRENT_BINARY_DIR}/test/Unit/lit.site.cfg
-      )
-  endif()
+  add_subdirectory(unittests)
+  list(APPEND CLANG_TEST_DEPS ClangUnitTests)
+  list(APPEND CLANG_TEST_PARAMS
+    clang_unit_site_config=${CMAKE_CURRENT_BINARY_DIR}/test/Unit/lit.site.cfg
+  )
   add_subdirectory(test)
   add_subdirectory(bindings/python/tests)