[flang] Harmonize standalone gtest support with other projects

[mgorny]

Summary

Use the CMake rules of unittest libraries via add_subdirectory()
when using standalone builds, similarly to how e.g. Clang does that.
Besides making things more consistent, I hope this also unblocks D137035
and effectively we'll be able to simplify things further and have LLVM
semi-automatically include gtest support for us in the future.

[phosek]

LGTM

[awarzynski]

Thanks for all the clean-up! LGTM

[mgorny]

Thanks!

flang/CMakeLists.txt

if (FLANG_STANDALONE_BUILD)
   get_errc_messages(LLVM_LIT_ERRC_MESSAGES)
 
 #Handle unittests when out-of-tree
   set(FLANG_GTEST_AVAIL 0)
   if (FLANG_INCLUDE_TESTS)
     set(UNITTEST_DIR ${LLVM_THIRD_PARTY_DIR}/unittest)
     if(EXISTS ${UNITTEST_DIR}/googletest/include/gtest/gtest.h)
       if (NOT TARGET llvm_gtest)
-        add_library(llvm_gtest
-          ${UNITTEST_DIR}/googletest/src/gtest-all.cc
-          ${UNITTEST_DIR}/googlemock/src/gmock-all.cc
-          )
-        target_include_directories(llvm_gtest
-          PUBLIC
-          "${UNITTEST_DIR}/googletest/include"
-          "${UNITTEST_DIR}/googlemock/include"
-
-          PRIVATE
-          "${UNITTEST_DIR}/googletest"
-          "${UNITTEST_DIR}/googlemock"
-          )
-         find_package(Threads)
-         target_link_libraries(llvm_gtest PUBLIC Threads::Threads)
-        add_library(llvm_gtest_main ${UNITTEST_DIR}/UnitTestMain/TestMain.cpp)
-        target_link_libraries(llvm_gtest_main PUBLIC llvm_gtest)
+        add_subdirectory(${UNITTEST_DIR} third-party/unittest)
       endif()
       set(FLANG_GTEST_AVAIL 1)
     else()
       message(WARNING
       "Unit-tests will be skipped as LLVM install does not include google-test related headers and libraries.")
       set(FLANG_GTEST_AVAIL 0)
     endif()
   endif()