[mlir] Fix -Wunused-variable in DebugImporter.cpp (NFC)

llvm-project/mlir/lib/Target/LLVMIR/DebugImporter.cpp:377:10:
error: unused variable '[_, inserted]' [-Werror,-Wunused-variable]
    auto [_, inserted] = dependentCache.try_emplace(
         ^
1 error generated.