[clang-fuzzer] Crash in `numStructUnionElements`

Hi, while testing clang by the fuzzing driver `clang-fuzzer`, it found a crashing case:

Version: 531fd45e9238d0485e3268aaf14ae15d01c7740f

Flags:
```
mkdir build
cd build
cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS="lld;clang;compiler-rt" ../llvm -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_BUILD_RUNTIME=Off -DLLVM_BUILD_INSTRUMENTED_COVERAGE=On -DCLANG_ENABLE_PROTO_FUZZER=ON
ninja clang-fuzzer
```

PoC:
```c
truct g{truct t o0}g[]{[0].o0[0]{}}
```

Reproduction:
`./bin/clang-fuzzer ./poc`

Crashing thread backtrace:
```
#0 0x000055555b758531 in (anonymous namespace)::InitListChecker::numStructUnionElements (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:1137

#1 0x000055555b7591f1 in (anonymous namespace)::InitListChecker::createInitListExpr (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:3525

#2 0x000055555b765ccb in (anonymous namespace)::InitListChecker::getStructuredSubobjectInit (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:3489

#3 0x000055555b7b81fb in (anonymous namespace)::InitListChecker::CheckDesignatedInitializer (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:2869

#4 0x000055555b7b8824 in (anonymous namespace)::InitListChecker::CheckDesignatedInitializer (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:3187

#5 0x000055555b7b77f0 in (anonymous namespace)::InitListChecker::CheckDesignatedInitializer (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:3398

#6 0x000055555b7a314f in (anonymous namespace)::InitListChecker::CheckArrayType (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:2231

#7 0x000055555b7a6e88 in (anonymous namespace)::InitListChecker::CheckListElementTypes (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:1463

#8 0x000055555b7a8659 in (anonymous namespace)::InitListChecker::CheckExplicitInitList (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:1345

#9 0x000055555b7aa27d in (anonymous namespace)::InitListChecker::InitListChecker (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:1108

#10 0x000055555b778c68 in TryListInitialization (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:5171

#11 0x000055555b77cb28 in clang::InitializationSequence::InitializeFrom (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaInit.cpp:6641

#12 0x000055555af5610f in clang::Sema::AddInitializerToDecl (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Sema/SemaDecl.cpp:13953

#13 0x000055555a03324e in clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/include/clang/AST/TemplateBase.h:44

#14 0x000055555a05ca4b in clang::Parser::ParseDeclGroup (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Parse/ParseDecl.cpp:2356

#15 0x0000555559fea9b0 in clang::Parser::ParseDeclOrFunctionDefInternal (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Parse/Parser.cpp:1187

#16 0x0000555559fec23b in clang::Parser::ParseDeclarationOrFunctionDefinition (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Parse/Parser.cpp:1209

#17 0x000055555a0024f0 in clang::Parser::ParseExternalDeclaration (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Parse/Parser.cpp:1032

#18 0x000055555a006d15 in clang::Parser::ParseTopLevelDecl (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Parse/Parser.cpp:745

#19 0x000055555a0079a0 in clang::Parser::ParseFirstTopLevelDecl (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Parse/Parser.cpp:601

#20 0x0000555559fbff86 in clang::ParseAST (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Parse/ParseAST.cpp:169

#21 0x0000555559ec1b28 in clang::ASTFrontendAction::ExecuteAction (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Frontend/FrontendAction.cpp:1432

#22 0x00005555564f8aa2 in clang::CodeGenAction::ExecuteAction (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/CodeGen/CodeGenAction.cpp:1109

#23 0x0000555559ed30d2 in clang::FrontendAction::Execute (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Frontend/FrontendAction.cpp:1312

#24 0x0000555559da3af8 in clang::CompilerInstance::ExecuteAction (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/lib/Frontend/CompilerInstance.cpp:1003

#25 0x0000555559d46f8e in clang::tooling::FrontendActionFactory::runInvocation (fuzz-binaries/clang-fuzzer)
443: bool clang::tooling::FrontendActionFactory::runInvocation(this = (clang::tooling::FrontendActionFactory * const)0x5555617751a0, Invocation = (std::shared_ptr)std::shared_ptr (empty) = {get() = 0x0}, Files = (clang::FileManager *)0x5555617715c0, PCHContainerOps = (std::shared_ptr)std::shared_ptr (empty) = {get() = 0x0}, DiagConsumer = (clang::DiagnosticConsumer *)0x7fffffff9990) {
|||:
---: }
at /usr/include/c++/9/bits/unique_ptr.h:154

#26 0x00005555564db128 in clang_fuzzer::HandleCXX (fuzz-binaries/clang-fuzzer)
23: void clang_fuzzer::HandleCXX(S = (const std::string &)"truct g{truct t o0}g[]{[0].o0[0]{}}", FileName = (const char *)0x5555558e74b7 "./test.cc", ExtraArgs = (const std::vector > &)std::vector of length 1, capacity 1 = {0x5555559613a3 "-O2"}) {
||||:
1386: template::type>
1387: typename enable_if::value>::type
1388: _M_enable_shared_from_this_with(_Yp*) noexcept
||||:
----: }
at /usr/include/c++/9/bits/shared_ptr_base.h:1388

#27 0x00005555564d3832 in LLVMFuzzerTestOneInput (fuzz-binaries/clang-fuzzer)
at /src/llvm/clang/tools/clang-fuzzer/ClangFuzzer.cpp:23
```

Start in clang/lib/Sema/SemaInit.cpp at InitListChecker::numStructUnionElements and reproduce the crash with clang-fuzzer using the provided C PoC and build flags. Trace the designated-initializer path shown in the backtrace; done means clang-fuzzer processes the input without crashing.

Written by the indexing model from the issue text.