[clang] Add missing deps edge: CodeGenAction.cpp.o -> GenVT.inc (PR #109306)

[Michał Górny via cfe-commits]

mgorny wrote:

This broke standalone clang builds:

```
-- Configuring done (6.4s)
CMake Error at /usr/lib/llvm/20/lib/cmake/llvm/AddLLVM.cmake:587 (add_dependencies):
  The dependency target "vt_gen" of target "obj.clangCodeGen" does not exist.
Call Stack (most recent call first):
  cmake/modules/AddClang.cmake:109 (llvm_add_library)
  lib/CodeGen/CMakeLists.txt:57 (add_clang_library)


CMake Error at /usr/lib/llvm/20/lib/cmake/llvm/AddLLVM.cmake:807 (add_dependencies):
  The dependency target "vt_gen" of target "clangCodeGen" does not exist.
Call Stack (most recent call first):
  cmake/modules/AddClang.cmake:109 (llvm_add_library)
  lib/CodeGen/CMakeLists.txt:57 (add_clang_library)
```

Since `vt_gen` target is part of LLVM, it's not available in standalone clang build. I guess the dep should be made conditional to `CLANG_BUILT_STANDALONE`.

https://github.com/llvm/llvm-project/pull/109306