[clang] -fsanitize=undefined causes Assertion `vlaSize && "no size for VLA!"' failed.

Reproducer:
https://godbolt.org/z/x8s5GhbE8
```cpp
void foo(unsigned short i) {
short a[i];
auto bar = [&iv = a](short value) { iv[0] = 0; };
bar(0);
}
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/CodeGen/CodeGenFunction.cpp:2420: clang::CodeGen::CodeGenFunction::VlaSizePair clang::CodeGen::CodeGenFunction::getVLASize(const clang::VariableArrayType*): Assertion `vlaSize && "no size for VLA!"' 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 -fsanitize=undefined
1. parser at end of file
2. Per-file LLVM IR generation
3. :3:14: Generating code for declaration 'foo(unsigned short)::(anonymous class)::operator()'
#0 0x00000000041e9638 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41e9638)
#1 0x00000000041e6a64 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41e6a64)
#2 0x000000000412b3b8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x000076bcc1642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x000076bcc16969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x000076bcc1642476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x000076bcc16287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x000076bcc162871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x000076bcc1639e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x0000000004752e21 clang::CodeGen::CodeGenFunction::getVLASize(clang::VariableArrayType const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4752e21)
#10 0x000000000453cb7c getArrayIndexingBound(clang::CodeGen::CodeGenFunction&, clang::Expr const*, clang::QualType&, clang::LangOptionsBase::StrictFlexArraysLevelKind) (.constprop.0) CGExpr.cpp:0:0
#11 0x000000000453d2ed clang::CodeGen::CodeGenFunction::EmitArraySubscriptExpr(clang::ArraySubscriptExpr const*, bool)::'lambda'(bool)::operator()(bool) const CGExpr.cpp:0:0
#12 0x0000000004547478 clang::CodeGen::CodeGenFunction::EmitArraySubscriptExpr(clang::ArraySubscriptExpr const*, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4547478)
#13 0x0000000004547ad3 clang::CodeGen::CodeGenFunction::EmitCheckedLValue(clang::Expr const*, clang::CodeGen::CodeGenFunction::TypeCheckKind) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4547ad3)
#14 0x0000000004559091 clang::CodeGen::CodeGenFunction::EmitBinaryOperatorLValue(clang::BinaryOperator const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4559091)
#15 0x000000000455f943 clang::CodeGen::CodeGenFunction::EmitLValueHelper(clang::Expr const*, clang::CodeGen::KnownNonNull_t) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x455f943)
#16 0x000000000455fd97 void llvm::function_ref::callback_fn(long) CGExpr.cpp:0:0
#17 0x00000000081aeb91 clang::StackExhaustionHandler::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x81aeb91)
#18 0x0000000004527e03 clang::CodeGen::CodeGenFunction::EmitLValue(clang::Expr const*, clang::CodeGen::KnownNonNull_t) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4527e03)
#19 0x000000000455800b clang::CodeGen::CodeGenFunction::EmitIgnoredExpr(clang::Expr const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x455800b)
#20 0x00000000046d7f87 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x46d7f87)
#21 0x00000000046e021c clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x46e021c)
#22 0x000000000474879e clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x474879e)
#23 0x000000000475a7c4 clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x475a7c4)
#24 0x00000000047c73d0 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47c73d0)
#25 0x00000000047c2284 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47c2284)
#26 0x00000000047cf12c clang::CodeGen::CodeGenModule::EmitDeferred() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47cf12c)
#27 0x00000000047d17d3 clang::CodeGen::CodeGenModule::Release() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x47d17d3)
#28 0x0000000004b31ebe (anonymous namespace)::CodeGeneratorImpl::HandleTranslationUnit(clang::ASTContext&) ModuleBuilder.cpp:0:0
#29 0x0000000004b2dec5 clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2dec5)
#30 0x00000000068c62bc clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x68c62bc)
#31 0x0000000004b2e6a8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b2e6a8)
#32 0x0000000004e1a3a5 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e1a3a5)
#33 0x0000000004d9bade clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4d9bade)
#34 0x0000000004f116bd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f116bd)
#35 0x0000000000dbfa50 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdbfa50)
#36 0x0000000000db65ba ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#37 0x0000000000db673d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#38 0x0000000004b97839 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#39 0x000000000412b854 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x412b854)
#40 0x0000000004b97e4f clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#41 0x0000000004b59162 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b59162)
#42 0x0000000004b5a10e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b5a10e)
#43 0x0000000004b61845 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b61845)
#44 0x0000000000dbbf79 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdbbf79)
#45 0x0000000000c6bf74 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc6bf74)
#46 0x000076bcc1629d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#47 0x000076bcc1629e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#48 0x0000000000db6055 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdb6055)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

Start by compiling the provided C++ reproducer with clang++ and -fsanitize=undefined to confirm the assertion. Then inspect CodeGenFunction.cpp at getVLASize and CGExpr.cpp around getArrayIndexingBound and EmitArraySubscriptExpr, as identified by the backtrace. Done means the reproducer no longer aborts during LLVM IR generation.

Written by the indexing model from the issue text.