[CMake] Add --gdb-index option to linker if split dwarf compiling

The option `--gdb-index` should be passed to linker if compiler is invoked
with `-gsplit-dwarf` option. This allows debugger to locate the appropriate
units quickly. See "Invocation" section here: https://gcc.gnu.org/wiki/DebugFission

Suggested by @dblaikie here: https://reviews.llvm.org/D68556#1698442

Differential Revision: https://reviews.llvm.org/D108776

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -901,6 +901,10 @@ if (LLVM_USE_SPLIT_DWARF AND
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR
       CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     add_compile_options(-gsplit-dwarf)
+  include(LLVMCheckLinkerFlag)
+  llvm_check_linker_flag(CXX "-Wl,--gdb-index" LINKER_SUPPORTS_GDB_INDEX)
+  append_if(LINKER_SUPPORTS_GDB_INDEX "-Wl,--gdb-index"
+    CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   endif()
 endif()