Assertion failed in VM flatbuffer serialization with empty NameLoc

```
iree-compile: iree/third_party/flatcc/src/runtime/builder.c:1492: flatcc_builder_ref_t _create_offset_vector_direct(flatcc_builder_t *, flatcc_builder_ref_t *, size_t, const flatbuffers_utype_t *): Assertion `types[i] == 0' failed.
Please report issues to https://github.com/openxla/iree/issues and include the crash backtrace.
Stack dump:
0. Program arguments: ./tools/iree-compile --iree-hal-target-backends=llvm-cpu --iree-input-type=onnx /home/stella/tmp/badloc.mlir -o /dev/null
#0 0x00007f7672c52c87 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/stella/src/iree/third_party/llvm-project/llvm/lib/Support/Unix/Signals.inc:723:13
#1 0x00007f7672c50eb0 llvm::sys::RunSignalHandlers() /home/stella/src/iree/third_party/llvm-project/llvm/lib/Support/Signals.cpp:106:18
#2 0x00007f7672c5334a SignalHandler(int) /home/stella/src/iree/third_party/llvm-project/llvm/lib/Support/Unix/Signals.inc:413:1
#3 0x00007f766eb9d520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x00007f766ebf1a7c __pthread_kill_implementation ./nptl/./nptl/pthread_kill.c:44:76
#5 0x00007f766ebf1a7c __pthread_kill_internal ./nptl/./nptl/pthread_kill.c:78:10
#6 0x00007f766ebf1a7c pthread_kill ./nptl/./nptl/pthread_kill.c:89:10
#7 0x00007f766eb9d476 gsignal ./signal/../sysdeps/posix/raise.c:27:6
#8 0x00007f766eb837f3 abort ./stdlib/./stdlib/abort.c:81:7
#9 0x00007f766eb8371b _nl_load_domain ./intl/./intl/loadmsgcat.c:1177:9
#10 0x00007f766eb94e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#11 0x00007f767582931e _create_offset_vector_direct /home/stella/src/iree/third_party/flatcc/src/runtime/builder.c:659:14
#12 0x00007f7675829bc0 flatcc_builder_create_union_vector_direct /home/stella/src/iree/third_party/flatcc/src/runtime/builder.c:1618:29
#13 0x00007f7675829bc0 flatcc_builder_create_union_vector /home/stella/src/iree/third_party/flatcc/src/runtime/builder.c:1606:13
#14 0x00007f7672d9cc6e iree_vm_LocationTypeDef_vec_create(flatcc_builder*, flatcc_builder_union_ref const*, unsigned long) /home/stella/src/iree-build/runtime/src/iree/../iree/schemas/bytecode_module_def_builder.h:299:1
#15 0x00007f7672d9cc6e mlir::iree_compiler::IREE::VM::LocationTable::finish() /home/stella/src/iree/compiler/src/iree/compiler/Dialect/VM/Target/Bytecode/DebugDatabaseBuilder.cpp:99:12
#16 0x00007f7672d9cc6e mlir::iree_compiler::IREE::VM::DebugDatabaseBuilder::build(mlir::iree_compiler::FlatbufferBuilder&) /home/stella/src/iree/compiler/src/iree/compiler/Dialect/VM/Target/Bytecode/DebugDatabaseBuilder.cpp:131:41
#17 0x00007f7672d8ef78 mlir::iree_compiler::IREE::VM::buildFlatBufferModule(mlir::iree_compiler::IREE::VM::TargetOptions, mlir::iree_compiler::IREE::VM::BytecodeTargetOptions, mlir::iree_compiler::IREE::VM::ModuleOp, llvm::MutableArrayRef, mlir::iree_compiler::FlatbufferBuilder&) /home/stella/src/iree/compiler/src/iree/compiler/Dialect/VM/Target/Bytecode/BytecodeModuleTarget.cpp:504:38
#18 0x00007f7672d8d1fa mlir::iree_compiler::IREE::VM::translateModuleToBytecode(mlir::iree_compiler::IREE::VM::ModuleOp, mlir::iree_compiler::IREE::VM::TargetOptions, mlir::iree_compiler::IREE::VM::BytecodeTargetOptions, llvm::raw_ostream&) /home/stella/src/iree/compiler/src/iree/compiler/Dialect/VM/Target/Bytecode/BytecodeModuleTarget.cpp:665:14
#19 0x00007f7672d8fc18 mlir::iree_compiler::IREE::VM::translateModuleToBytecode(mlir::ModuleOp, mlir::iree_compiler::IREE::VM::TargetOptions, mlir::iree_compiler::IREE::VM::BytecodeTargetOptions, llvm::raw_ostream&) /home/stella/src/iree/compiler/src/iree/compiler/Dialect/VM/Target/Bytecode/BytecodeModuleTarget.cpp:687:10
#20 0x00007f767374a7e8 mlir::iree_compiler::ConstEval::InMemoryCompiledBinary::translateFromModule(mlir::ModuleOp) /home/stella/src/iree/compiler/src/iree/compiler/ConstEval/Runtime.cpp:470:14
#21 0x00007f7672d8820b mlir::LogicalResult::failed() const /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:44:33
#22 0x00007f7672d8820b mlir::failed(mlir::LogicalResult) /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:72:58
#23 0x00007f7672d8820b mlir::iree_compiler::ConstEval::(anonymous namespace)::JitGlobalsPass::runOnOperation() /home/stella/src/iree/compiler/src/iree/compiler/ConstEval/JitGlobals.cpp:534:9
#24 0x00007f7672ddff75 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int)::$_7::operator()() const /home/stella/src/iree/third_party/llvm-project/mlir/lib/Pass/Pass.cpp:0:17
#25 0x00007f7672ddff75 void llvm::function_ref::callback_fn(long) /home/stella/src/iree/third_party/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:12
#26 0x00007f7672ddff75 llvm::function_ref::operator()() const /home/stella/src/iree/third_party/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:12
#27 0x00007f7672ddff75 void mlir::MLIRContext::executeAction(llvm::function_ref, llvm::ArrayRef, mlir::Pass&) /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/IR/MLIRContext.h:275:7
#28 0x00007f7672ddff75 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int) /home/stella/src/iree/third_party/llvm-project/mlir/lib/Pass/Pass.cpp:507:21
#29 0x00007f7672de06f8 mlir::LogicalResult::failed() const /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:44:33
#30 0x00007f7672de06f8 mlir::failed(mlir::LogicalResult) /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:72:58
#31 0x00007f7672de06f8 mlir::detail::OpToOpPassAdaptor::runPipeline(mlir::OpPassManager&, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int, mlir::PassInstrumentor*, mlir::PassInstrumentation::PipelineParentInfo const*) /home/stella/src/iree/third_party/llvm-project/mlir/lib/Pass/Pass.cpp:579:9
#32 0x00007f7672de4f21 mlir::LogicalResult llvm::function_ref::callback_fn(long, mlir::OpPassManager&, mlir::Operation*) /home/stella/src/iree/third_party/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:5
#33 0x00007f767422e4bd mlir::iree_compiler::IREE::Util::(anonymous namespace)::FixedPointIteratorPass::runOnOperation() /home/stella/src/iree/compiler/src/iree/compiler/Dialect/Util/Transforms/FixedPointIterator.cpp:0:0
#34 0x00007f7672ddff75 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int)::$_7::operator()() const /home/stella/src/iree/third_party/llvm-project/mlir/lib/Pass/Pass.cpp:0:17
#35 0x00007f7672ddff75 void llvm::function_ref::callback_fn(long) /home/stella/src/iree/third_party/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:12
#36 0x00007f7672ddff75 llvm::function_ref::operator()() const /home/stella/src/iree/third_party/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:12
#37 0x00007f7672ddff75 void mlir::MLIRContext::executeAction(llvm::function_ref, llvm::ArrayRef, mlir::Pass&) /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/IR/MLIRContext.h:275:7
#38 0x00007f7672ddff75 mlir::detail::OpToOpPassAdaptor::run(mlir::Pass*, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int) /home/stella/src/iree/third_party/llvm-project/mlir/lib/Pass/Pass.cpp:507:21
#39 0x00007f7672de06f8 mlir::LogicalResult::failed() const /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:44:33
#40 0x00007f7672de06f8 mlir::failed(mlir::LogicalResult) /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:72:58
#41 0x00007f7672de06f8 mlir::detail::OpToOpPassAdaptor::runPipeline(mlir::OpPassManager&, mlir::Operation*, mlir::AnalysisManager, bool, unsigned int, mlir::PassInstrumentor*, mlir::PassInstrumentation::PipelineParentInfo const*) /home/stella/src/iree/third_party/llvm-project/mlir/lib/Pass/Pass.cpp:579:9
#42 0x00007f7672de2a99 mlir::PassManager::run(mlir::Operation*) /home/stella/src/iree/third_party/llvm-project/mlir/lib/Pass/Pass.cpp:0:0
#43 0x00007f7672badb27 mlir::LogicalResult::failed() const /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:44:33
#44 0x00007f7672badb27 mlir::failed(mlir::LogicalResult) /home/stella/src/iree/third_party/llvm-project/mlir/include/mlir/Support/LogicalResult.h:72:58
#45 0x00007f7672badb27 mlir::iree_compiler::embed::(anonymous namespace)::Invocation::runPipeline(iree_compiler_pipeline_t) /home/stella/src/iree/compiler/src/iree/compiler/API/Internal/CompilerDriver.cpp:958:7
#46 0x00007f7672badb27 ireeCompilerInvocationPipeline /home/stella/src/iree/compiler/src/iree/compiler/API/Internal/CompilerDriver.cpp:1388:23
#47 0x00007f7672daa665 mlir::iree_compiler::runIreecMain(int, char**)::$_2::operator()(iree_compiler_source_t*) const /home/stella/src/iree/compiler/src/iree/compiler/Tools/iree_compile_lib.cc:247:11
#48 0x00007f7672da9f10 mlir::iree_compiler::runIreecMain(int, char**) /home/stella/src/iree/compiler/src/iree/compiler/Tools/iree_compile_lib.cc:0:10
#49 0x00007f766eb84d90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#50 0x00007f766eb84e40 call_init ./csu/../csu/libc-start.c:128:20
#51 0x00007f766eb84e40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#52 0x000055759370e6b5 _start (./tools/iree-compile+0x16b5)
```

Repro:

```
./tools/iree-compile --iree-hal-target-backends=llvm-cpu --iree-input-type=onnx ~/tmp/badloc.mlir -o /dev/null
```

badloc.mlir:

```
#loc1 = loc("graph:gemm_activation_fusion")
module {
func.func @IreeExecutionProvider_IREE_0(%arg0: !torch.vtensor<[3,5],f32> loc("graph:gemm_activation_fusion"), %arg1: !torch.vtensor<[5,4],f32> loc("graph:gemm_activation_fusion"), %arg2: !torch.vtensor<[1,4],f32> loc("graph:gemm_activation_fusion")) -> !torch.vtensor<[3,4],f32> attributes {torch.onnx_meta.ir_version = 0 : si64, torch.onnx_meta.opset_version = 9 : si64, torch.onnx_meta.opset_versions = {ai.onnx.ml = 4 : si64, ai.onnx.preview.training = 1 : si64, ai.onnx.training = 1 : si64, com.microsoft = 1 : si64, com.microsoft.experimental = 1 : si64, com.microsoft.nchwc = 1 : si64, com.ms.internal.nhwc = 9 : si64, org.pytorch.aten = 1 : si64}, torch.onnx_meta.producer_name = "", torch.onnx_meta.producer_version = ""} {
%0 = torch.operator "onnx.Gemm"(%arg0, %arg1, %arg2) {torch.onnx.alpha = 1.000000e+00 : f32, torch.onnx.beta = 1.000000e+00 : f32, torch.onnx.transA = 0 : si64, torch.onnx.transB = 0 : si64} : (!torch.vtensor<[3,5],f32>, !torch.vtensor<[5,4],f32>, !torch.vtensor<[1,4],f32>) -> !torch.vtensor<[3,4],f32> loc(#loc2)
%1 = torch.operator "onnx.LeakyRelu"(%0) {torch.onnx.alpha = 0.00999999977 : f32} : (!torch.vtensor<[3,4],f32>) -> !torch.vtensor<[3,4],f32> loc(#loc2)
return %1 : !torch.vtensor<[3,4],f32> loc(#loc1)
} loc(#loc1)
} loc(#loc)
#loc = loc(unknown)
#loc2 = loc("")
```

I've patched the caller to not create such locs and verified that it works.