[clang] Assertion `CGF.getContext().hasSameUnqualifiedType(E->getSubExpr()->getType(), E->getType()) && "Implicit cast types must be compatible"' failed.

Reproducer:
https://godbolt.org/z/jzTf8aMsM
```c
struct h {
int b;
int a[] __attribute__((counted_by(b)));
} p;

void foo(void) {
struct h {
int b;
int a[] __attribute__((counted_by(b)));
} x;

p = x;
}
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/CodeGen/CGExprAgg.cpp:986: void {anonymous}::AggExprEmitter::VisitCastExpr(clang::CastExpr*): Assertion `CGF.getContext().hasSameUnqualifiedType(E->getSubExpr()->getType(), E->getType()) && "Implicit cast types must be compatible"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0. Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -g -o /app/output.s -mllvm --x86-asm-syntax=intel -fno-verbose-asm -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -x c -std=c23
1. parser at end of file
2. :6:6: LLVM IR generation of declaration 'foo'
3. :6:6: Generating code for declaration 'foo'
#0 0x00000000042021d8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42021d8)
#1 0x00000000041ff604 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41ff604)
#2 0x0000000004143778 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x000070f077a42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x000070f077a969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x000070f077a42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x000070f077a287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x000070f077a2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x000070f077a39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x000000000458d85c (anonymous namespace)::AggExprEmitter::VisitCastExpr(clang::CastExpr*) CGExprAgg.cpp:0:0
#10 0x00000000045894f1 (anonymous namespace)::AggExprEmitter::Visit(clang::Expr*) CGExprAgg.cpp:0:0
#11 0x000000000458a1ee clang::CodeGen::CodeGenFunction::EmitAggExpr(clang::Expr const*, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x458a1ee)
#12 0x000000000458aace (anonymous namespace)::AggExprEmitter::VisitBinAssign(clang::BinaryOperator const*) CGExprAgg.cpp:0:0
#13 0x0000000004589bab (anonymous namespace)::AggExprEmitter::Visit(clang::Expr*) CGExprAgg.cpp:0:0
#14 0x000000000458a1ee clang::CodeGen::CodeGenFunction::EmitAggExpr(clang::Expr const*, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x458a1ee)
#15 0x0000000004542601 clang::CodeGen::CodeGenFunction::EmitAnyExpr(clang::Expr const*, clang::CodeGen::AggValueSlot, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4542601)
#16 0x0000000004573a6d clang::CodeGen::CodeGenFunction::EmitIgnoredExpr(clang::Expr const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4573a6d)
#17 0x00000000046f4257 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x46f4257)
#18 0x00000000046fc53c clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x46fc53c)
#19 0x0000000004764a8e clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4764a8e)
#20 0x0000000004776ae4 clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4776ae4)
#21 0x00000000047e3950 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47e3950)
#22 0x00000000047de824 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47de824)
#23 0x00000000047df6c0 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47df6c0)
#24 0x00000000047e9ea3 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#25 0x0000000004b4f1e9 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#26 0x0000000004b3e314 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b3e314)
#27 0x00000000068e74e4 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68e74e4)
#28 0x0000000004b4abe8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b4abe8)
#29 0x0000000004e38265 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e38265)
#30 0x0000000004db92fe clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4db92fe)
#31 0x0000000004f2fcfd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f2fcfd)
#32 0x0000000000dc2b50 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdc2b50)
#33 0x0000000000db96ba ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#34 0x0000000000db983d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#35 0x0000000004bb3d99 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#36 0x0000000004143c14 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4143c14)
#37 0x0000000004bb43af clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#38 0x0000000004b756c2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b756c2)
#39 0x0000000004b7666e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b7666e)
#40 0x0000000004b7dda5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b7dda5)
#41 0x0000000000dbf079 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdbf079)
#42 0x0000000000c6ee04 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc6ee04)
#43 0x000070f077a29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#44 0x000070f077a29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#45 0x0000000000db9155 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb9155)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

Reproduce the C23 case from the issue, then inspect llvm/tools/clang/lib/CodeGen/CGExprAgg.cpp around AggExprEmitter::VisitCastExpr and the assertion at line 986. Confirm the assignment no longer aborts during code generation and add coverage for this reproducer in the relevant Clang test suite.

Written by the indexing model from the issue text.