[CMake][unittests] Teach export_executable_symbols about ALIAS targets (#167932)

DynamicLibraryTests has a workaround for AIX that calls
export_executable_symbols after add_llvm_unittest, instead of just
passing the `EXPORT_SYMBOLS` switch. At this point, the
default_gtest(_main) libraries is added to the linked library set.
However, default_gtest(_main) are ALIAS libraries. This PR replaces the
ALIAS libraries with the ALIASED_TARGET before passing it to
extract_symbols.py.

Fixes failure of the clang-ppc64-aix buildbot after #164794.

Author: Meinersbur

llvm/cmake/modules/AddLLVM.cmake

@@ -1359,6 +1359,14 @@ function(export_executable_symbols target)
     while(NOT "${new_libs}" STREQUAL "")
       foreach(lib ${new_libs})
         if(TARGET ${lib})
+          # If this is a ALIAS target, continue with its aliasee instead.
+          get_target_property(aliased_lib ${lib} ALIASED_TARGET)
+          if(aliased_lib)
+             set(new_libs ${lib_aliased_target})
+             list(APPEND newer_libs ${aliased_lib})
+             continue()
+          endif()
+
           get_target_property(lib_type ${lib} TYPE)
           if("${lib_type}" STREQUAL "STATIC_LIBRARY")
             list(APPEND static_libs ${lib})