[mlir] Fix FunctionOpInterface impl for external func (#124693)

For function declarations (i.e. func op has no entry block), the
FunctionOpInterface method `insertArgument` and `eraseArgument` will
cause segfault. This PR guards against manipulation of empty entry block
by checking whether func op is external.

An example can be seen in https://github.com/google/heir/pull/1324

The segfault trace

```
 #1 0x0000560f1289d9db PrintStackTraceSignalHandler(void*) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:874:1
 #2 0x0000560f1289b116 llvm::sys::RunSignalHandlers() /proc/self/cwd/external/llvm-project/llvm/lib/Support/Signals.cpp:105:5
 #3 0x0000560f1289e145 SignalHandler(int) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:415:1
 #4 0x00007f829a3d9520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #5 0x0000560f1257f8bc void __gnu_cxx::new_allocator<mlir::BlockArgument>::construct<mlir::BlockArgument, mlir::BlockArgument>(mlir::BlockArgument*, mlir::BlockArgument&&) /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/ext/new_allocator.h:162:23
 #6 0x0000560f1257f84d void std::allocator_traits<std::allocator<mlir::BlockArgument> >::construct<mlir::BlockArgument, mlir::BlockArgument>(std::allocator<mlir::BlockArgument>&, mlir::BlockArgument*, mlir::BlockArgument&&) /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/alloc_traits.h:520:2
 #7 0x0000560f12580498 void std::vector<mlir::BlockArgument, std::allocator<mlir::BlockArgument> >::_M_insert_aux<mlir::BlockArgument>(__gnu_cxx::__normal_iterator<mlir::BlockArgument*, std::vector<mlir::BlockArgument, std::allocator<mlir::BlockArgument> > >, mlir::BlockArgument&&) /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/vector.tcc:405:7
 #8 0x0000560f1257cf7e std::vector<mlir::BlockArgument, std::allocator<mlir::BlockArgument> >::insert(__gnu_cxx::__normal_iterator<mlir::BlockArgument const*, std::vector<mlir::BlockArgument, std::allocator<mlir::BlockArgument> > >, mlir::BlockArgument const&) /usr/lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/vector.tcc:154:6
 #9 0x0000560f1257b349 mlir::Block::insertArgument(unsigned int, mlir::Type, mlir::Location) /proc/self/cwd/external/llvm-project/mlir/lib/IR/Block.cpp:178:13
#10 0x0000560f123d2a1c mlir::function_interface_impl::insertFunctionArguments(mlir::FunctionOpInterface, llvm::ArrayRef<unsigned int>, mlir::TypeRange, llvm::ArrayRef<mlir::DictionaryAttr>, llvm::ArrayRef<mlir::Location>, unsigned int, mlir::Type) /proc/self/cwd/external/llvm-project/mlir/lib/Interfaces/FunctionInterfaces.cpp:232:11
#11 0x0000560f0be6b727 mlir::detail::FunctionOpInterfaceTrait<mlir::func::FuncOp>::insertArguments(llvm::ArrayRef<unsigned int>, mlir::TypeRange, llvm::ArrayRef<mlir::DictionaryAttr>, llvm::ArrayRef<mlir::Location>) /proc/self/cwd/bazel-out/k8-dbg/bin/external/llvm-project/mlir/include/mlir/Interfaces/FunctionInterfaces.h.inc:809:7
#12 0x0000560f0be6b536 mlir::detail::FunctionOpInterfaceTrait<mlir::func::FuncOp>::insertArgument(unsigned int, mlir::Type, mlir::DictionaryAttr, mlir::Location) /proc/self/cwd/bazel-out/k8-dbg/bin/external/llvm-project/mlir/include/mlir/Interfaces/FunctionInterfaces.h.inc:796:7
```
1 file changed
tree: 33b97049cf2025e6a36976d158bd51f44d714525
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
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