[cmake] Build gtest from LLVM when building standalone

[mgorny]

Summary

Include the gtest utility directory from LLVM sources when performing
a stand-alone build of LLDB. This is necessary to have a correct gtest
library to link tests against, as the one used by LLVM is not installed
(and not supposed to be). This is the same approach as used in clang.

[beanz]

LGTM.

unittests/CMakeLists.txt

 add_custom_target(LLDBUnitTests)
 set_target_properties(LLDBUnitTests PROPERTIES FOLDER "LLDB tests")
 
 include_directories(${LLDB_SOURCE_ROOT})
 
 set(LLDB_GTEST_COMMON_INCLUDE ${CMAKE_CURRENT_SOURCE_DIR}/gtest_common.h)
 if (MSVC)
   list(APPEND LLVM_COMPILE_FLAGS /FI ${LLDB_GTEST_COMMON_INCLUDE})
 else ()
   list(APPEND LLVM_COMPILE_FLAGS -include ${LLDB_GTEST_COMMON_INCLUDE})
 endif ()
 
 include(${LLDB_PROJECT_ROOT}/cmake/LLDBDependencies.cmake)
 
+if (LLDB_BUILT_STANDALONE)
+  # Build the gtest library needed for unittests, if we have LLVM sources
+  # handy.
+  if (EXISTS ${LLVM_MAIN_SRC_DIR}/utils/unittest)
+    add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/unittest utils/unittest)
+  endif()
+endif()
+
 function(add_lldb_unittest test_name)
   add_unittest(LLDBUnitTests
     ${test_name}
     ${ARGN}
     )
 
   add_custom_command(
     TARGET ${test_name}