[CMake][Release] Build with -ffat-lto-objects

Fixes #133580

Author: tstellar

clang/cmake/caches/Release.cmake

@@ -102,7 +102,7 @@ if (LLVM_RELEASE_ENABLE_LTO)
   # FIXME: We can't use LLVM_ENABLE_LTO=Thin here, because it causes the CMake
   # step for the libcxx build to fail.  CMAKE_INTERPROCEDURAL_OPTIMIZATION does
   # enable ThinLTO, though.
-  set(RUNTIMES_CMAKE_ARGS "-DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON -DLLVM_ENABLE_LLD=ON" CACHE STRING "")
+  set(RUNTIMES_CMAKE_ARGS "-DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON -DLLVM_ENABLE_LLD=ON -DLLVM_ENABLE_FATLTO=ON" CACHE STRING "")
 endif()
 
 # Stage 1 Common Config
@@ -144,3 +144,6 @@ set_final_stage_var(CPACK_GENERATOR "TXZ" STRING)
 set_final_stage_var(CPACK_ARCHIVE_THREADS "0" STRING)
 
 set_final_stage_var(LLVM_USE_STATIC_ZSTD "ON" BOOL)
+if (LLVM_RELEASE_ENABLE_LTO)
+  set_final_stage_var(LLVM_ENABLE_FATLTO "ON" BOOL)
+endif()