[CMake][unittests] Teach export_executable_symbols about ALIAS targets

[Meinersbur]

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.

[daltenty]

LGTM, thanks!

[madanial0]

Thanks for the quick fix!

[llvm-ci]

LLVM Buildbot has detected a new failure on builder reverse-iteration running on hexagon-build-03 while building llvm at step 6 "check_all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/110/builds/6292

Here is the relevant piece of the build log for the reference

Step 6 (check_all) failure: test (failure)
******************** TEST 'Clang :: Interpreter/dynamic-library.cpp' FAILED ********************
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 17
cat /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.src/clang/test/Interpreter/dynamic-library.cpp | env LD_LIBRARY_PATH=/local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.src/clang/test/Interpreter/Inputs:$LD_LIBRARY_PATH /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.obj/bin/clang-repl | /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.obj/bin/FileCheck /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.src/clang/test/Interpreter/dynamic-library.cpp
# executed command: cat /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.src/clang/test/Interpreter/dynamic-library.cpp
# .---command stdout------------
# | // REQUIRES: host-supports-jit, x86_64-linux
# | 
# | // To generate libdynamic-library-test.so :
# | // clang -xc++ -o libdynamic-library-test.so -fPIC -shared
# | //
# | // extern "C" {
# | //
# | // int ultimate_answer = 0;
# | // 
# | // int calculate_answer() {
# | //   ultimate_answer = 42;
# | //   return 5;
# | // }
# | //
# | // }
# | 
# | // RUN: cat %s | env LD_LIBRARY_PATH=%S/Inputs:$LD_LIBRARY_PATH clang-repl | FileCheck %s
# | 
# | extern "C" int printf(const char* format, ...);
# | 
# | extern "C" int ultimate_answer;
# | extern "C" int calculate_answer();
# | 
# | %lib libdynamic-library-test.so
# | 
# | printf("Return value: %d\n", calculate_answer());
# | // CHECK: Return value: 5
# | 
# | printf("Variable: %d\n", ultimate_answer);
# | // CHECK-NEXT: Variable: 42
# | 
# | %quit
# `-----------------------------
# executed command: env 'LD_LIBRARY_PATH=/local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.src/clang/test/Interpreter/Inputs:$LD_LIBRARY_PATH' /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.obj/bin/clang-repl
# .---command stderr------------
# | /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.obj/bin/clang-repl: error while loading shared libraries: libc++.so.1: cannot open shared object file: No such file or directory
# `-----------------------------
# error: command failed with exit status: 127
# executed command: /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.obj/bin/FileCheck /local/mnt/workspace/bots/hexagon-build-03/reverse-iteration/llvm.src/clang/test/Interpreter/dynamic-library.cpp
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
...