[flang] Fix unittests in standalone build for third-party move

[mgorny]

Summary

Update external unittest rules for flang standalone builds to account
for the unittest components (gtest, gmock) being moved to third-party
directory, in a11cd0d94ed3cabf0998a0289aead05da94c86eb.

[awarzynski]

Makes sense, thanks for the clean-up! LGTM

[mgorny]

Thanks!

flang/CMakeLists.txt

if (FLANG_STANDALONE_BUILD)
 
   option(FLANG_INCLUDE_TESTS
          "Generate build targets for the Flang unit tests."
          ON)
 
   get_errc_messages(LLVM_LIT_ERRC_MESSAGES)
 
 #Handle unittests when out-of-tree
-#LLVM_BUILD_MAIN_SRC_DIR - Path to llvm source when out-of-tree.
   set(FLANG_GTEST_AVAIL 0)
   if (FLANG_INCLUDE_TESTS)
-    set(UNITTEST_DIR ${LLVM_BUILD_MAIN_SRC_DIR}/utils/unittest)
+    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