Additional patch to Clangs CMake code is required in case LLVM was built with `LLVM_LINK_LLVM_DYLIB=ON`

### Description

Additional patches to Clangs CMake code are required in case LLVM was built with `LLVM_LINK_LLVM_DYLIB=ON`, similar to the patches that were done in ROOT here:
  * https://github.com/root-project/root/pull/18386

```diff
diff --git a/interpreter/llvm-project/clang/CMakeLists.txt b/interpreter/llvm-project/clang/CMakeLists.txt
index 5f2b7f064d..3a2e593aa7 100644
--- a/interpreter/llvm-project/clang/CMakeLists.txt
+++ b/interpreter/llvm-project/clang/CMakeLists.txt
@@ -29,6 +29,15 @@ if(CLANG_BUILT_STANDALONE)
   endif()

   find_package(LLVM REQUIRED HINTS "${LLVM_CMAKE_DIR}")
+
+  # Disable linking against shared LLVM. We have to do this after
+  # find_package(LLVM), which overwrites LLVM_LINK_LLVM_DYLIB to what it was
+  # set when building LLVM. Clang needs to link LLVM statically, otherwise it
+  # won't work for Cling. By doing forcing this, we can build Clang on top of
+  # existing LLVM builds even they were built with LLVM_LINK_LLVM_DYLIB=ON,
+  # which is usually the case for system packages.
+  set(LLVM_LINK_LLVM_DYLIB FALSE)
+
   list(APPEND CMAKE_MODULE_PATH "${LLVM_DIR}")

   # Turn into CACHE PATHs for overwritting
```

Where should this patch go? Would it be reasonable to do it in our fork of LLVM and then sync ROOT with it?
https://github.com/root-project/llvm-project

@hahnjo, your advice would be very appreciated here!

### Reproducer

* take LLVM build with `LLVM_LINK_LLVM_DYLIB=ON` (usually the case for the system packages)
* build patched Clang for ROOT standalone
* try to build ROOT with `builtin_llvm=OFF` and `builtin_clang=OFF`

Only with the patch in the Clang CMakeLists.txt, it works for me.

### Additional context

See also this issue for some context on why it is necessary to statically link against LLVM:
  * https://github.com/root-project/root/issues/17698

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