[CMake] Force LLVM discovery via LLVMConfig.cmake (CONFIG mode) (KhronosGroup#3598)

This makes LLVM detection deterministic and ensures we use LLVM’s
supported CMake integration (`LLVMConfig.cmake`) rather than potentially
picking up an environment-provided `FindLLVM.cmake` module.

* Out-of-tree builds require `LLVMConfig.cmake` to be discoverable (set
`LLVM_DIR` / `CMAKE_PREFIX_PATH` if needed).
* In-tree builds are unaffected (the external `find_package(LLVM ...)`
path isn’t used there).

Author: vmaksimo

CMakeLists.txt

@@ -79,7 +79,7 @@ if(LLVM_SPIRV_BUILD_EXTERNAL)
     )
   endif(LLVM_SPIRV_INCLUDE_TESTS)
 
-  find_package(LLVM ${BASE_LLVM_VERSION} REQUIRED
+  find_package(LLVM ${BASE_LLVM_VERSION} REQUIRED CONFIG
     COMPONENTS
       Analysis
       BitReader