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

Update external unittest rules for flang standalone builds to account
for the unittest components (gtest, gmock) being moved to `third-party`
directory, in a11cd0d.

Differential Revision: https://reviews.llvm.org/D138562

Author: mgorny

flang/CMakeLists.txt

@@ -148,10 +148,9 @@ if (FLANG_STANDALONE_BUILD)
   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