Assertion failure numElts > 1 && "illegal vector length" in isLegalVectorType (SwiftCallingConv.cpp) on a single-element vector type passed via swiftcall

## Description

Clang crashes lowering a swiftcall-ABI struct/parameter containing a single-element vector type, e.g. `__attribute__((vector_size(8)))` on `long long`.

## Reproduction

Repro.c

```c
typedef long long CVectorOne __attribute__((vector_size(8)));

struct VectorBox { CVectorOne v; };

__attribute__((swiftcall)) void takeVectorBox(struct VectorBox box) {}
```

`$ clang -target x86_64-apple-macosx13.0 -c Repro.c`

```
Assertion failed: (numElts > 1 && "illegal vector length"), function isLegalVectorType, file SwiftCallingConv.cpp, line 696.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and dumped files.
Stack dump:
0. Program arguments: /Users/hiroshi/upstream-llvm/llvm-project/build/bin/clang -target x86_64-apple-macosx13.0 -c Repro.c
1. parser at end of file
2. Repro.c:5:33: LLVM IR generation of declaration 'takeVectorBox'
3. Repro.c:5:33: Generating code for declaration 'takeVectorBox'
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0 clang-24 0x000000010716ed2c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 56
1 clang-24 0x000000010716c134 llvm::sys::RunSignalHandlers() + 204
2 clang-24 0x000000010716db70 llvm::sys::CleanupOnSignal(unsigned long) + 304
3 clang-24 0x00000001070ab1dc CrashRecoverySignalHandler(int) + 164
4 libsystem_platform.dylib 0x0000000192d64144 _sigtramp + 56
5 libsystem_pthread.dylib 0x0000000192d5997c pthread_kill + 296
6 libsystem_c.dylib 0x0000000192c5c458 abort + 148
7 libsystem_c.dylib 0x0000000192c5b6a0 err + 0
8 clang-24 0x000000010a184368 clang::CodeGen::swiftcall::SwiftAggLowering::addLegalTypedData(llvm::Type*, clang::CharUnits, clang::CharUnits) (.cold.1) + 0
9 clang-24 0x0000000107a7ff38 clang::CodeGen::swiftcall::legalizeVectorType(clang::CodeGen::CodeGenModule&, clang::CharUnits, llvm::VectorType*, llvm::SmallVectorImpl&) + 912
10 clang-24 0x0000000107a7f3cc clang::CodeGen::swiftcall::SwiftAggLowering::addTypedData(llvm::Type*, clang::CharUnits, clang::CharUnits) + 160
11 clang-24 0x0000000107a7f8fc clang::CodeGen::swiftcall::SwiftAggLowering::addTypedData(clang::RecordDecl const*, clang::CharUnits, clang::ASTRecordLayout const&) + 728
12 clang-24 0x0000000107a81828 classifyType(clang::CodeGen::CodeGenModule&, clang::CanQual, bool) + 516
13 clang-24 0x0000000107a81964 clang::CodeGen::swiftcall::computeABIInfo(clang::CodeGen::CodeGenModule&, clang::CodeGen::CGFunctionInfo&) + 96
14 clang-24 0x00000001077450dc clang::CodeGen::CodeGenTypes::findOrInsertCGFunctionInfo(bool, bool, bool, unsigned int, clang::FunctionType::ExtInfo const&, llvm::ArrayRef, clang::CodeGen::RequiredArgs, clang::CanQual, llvm::ArrayRef>) + 676
15 clang-24 0x0000000107741064 clang::CodeGen::CodeGenTypes::arrangeLLVMFunctionInfo(clang::CanQual, clang::CodeGen::FnInfoOpts, llvm::ArrayRef>, clang::FunctionType::ExtInfo, llvm::ArrayRef, clang::CodeGen::RequiredArgs, clang::FunctionDecl const*) + 264
16 clang-24 0x0000000107742ea8 clang::CodeGen::CodeGenTypes::arrangeFunctionDeclaration(clang::GlobalDecl) + 1076
17 clang-24 0x00000001079e6c70 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) + 116
18 clang-24 0x00000001079dfb28 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) + 424
19 clang-24 0x00000001079e3f9c clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) + 1768
20 clang-24 0x00000001079de1a4 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) + 268
21 clang-24 0x0000000107a6644c (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) + 144
22 clang-24 0x00000001079ae3f8 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) + 172
23 clang-24 0x00000001089b68b0 clang::ParseAST(clang::Sema&, bool, bool) + 580
24 clang-24 0x0000000107db7c80 clang::FrontendAction::Execute() + 48
25 clang-24 0x0000000107d465f0 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) + 1144
26 clang-24 0x0000000107e33dbc clang::ExecuteCompilerInvocation(clang::CompilerInstance*) + 452
27 clang-24 0x000000010435d9c0 cc1_main(llvm::ArrayRef, char const*, void*) + 1424
28 clang-24 0x000000010435b968 ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) + 1220
29 clang-24 0x000000010435d250 int llvm::function_ref&)>::callback_fn(long, llvm::SmallVectorImpl&) + 60
30 clang-24 0x0000000107b95694 void llvm::function_ref::callback_fn>, std::__1::basic_string, std::__1::allocator>*, bool*) const::$_0>(long) + 32
31 clang-24 0x00000001070aaed8 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) + 168
32 clang-24 0x0000000107b94fe4 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__1::basic_string, std::__1::allocator>*, bool*) const + 288
33 clang-24 0x0000000107b5b80c clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const + 728
34 clang-24 0x0000000107b5bed0 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const + 1328
35 clang-24 0x0000000107b7ce88 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) + 328
36 clang-24 0x000000010435a9c0 clang_main(int, char**, llvm::ToolContext const&) + 4340
37 clang-24 0x0000000104367e30 main + 92
38 dyld 0x0000000192983e80 start + 6688
clang: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 24.0.0git (git@github.com:llvm/llvm-project.git 3018c32171d439dec1b76ef961cc60c0d8db2c6c)
Target: x86_64-apple-macosx13.0
Thread model: posix
InstalledDir: /Users/hiroshi/upstream-llvm/llvm-project/build/bin
Build config: +assertions
clang: note: diagnostic msg:
```

Start with the Repro.c example and run the provided clang command for the x86_64 Apple target. Then inspect SwiftCallingConv.cpp around isLegalVectorType and legalizeVectorType; done means this single-element vector in the swiftcall struct no longer triggers the assertion, with regression coverage for the reproducer.

Written by the indexing model from the issue text.