Clang ICE: constexpr evaluation of ext_vector_type assignment triggers APValue::getVectorElt assertion

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### Link for quick verification: https://godbolt.org/z/1PxjvMed1
```c
#include
typedef __attribute__((ext_vector_type(64))) __int128 vtype ;

constexpr vtype fill() {
vtype result;
for (size_t i = 0; i < 64; ++i) {
result[i] = i;
}
return result;
}
```
Go back to `clang 20.1.0`.
The code uses a `constexpr` function that initializes a Clang vector extension type `(__attribute__((ext_vector_type(64))))` of `__int128` via indexed assignment. This may be an inconsistency between vector element lvalue handling and APValue representation during constexpr evaluation.

## Stack dump:
```console
clang++: /root/llvm-project/llvm/tools/clang/include/clang/AST/APValue.h:564: clang::APValue& clang::APValue::getVectorElt(unsigned int): Assertion `isVector() && "Invalid accessor"' 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
1. parser at end of file
2. :5:24: parsing function body 'fill'
#0 0x00000000042ebc28 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42ebc28)
#1 0x00000000042e9054 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42e9054)
#2 0x000000000422de38 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x0000727ea3a42520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x0000727ea3a969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x0000727ea3a42476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x0000727ea3a287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x0000727ea3a2871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
#8 0x0000727ea3a39e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#9 0x0000000007cdcbe7 clang::APValue::getVectorElt(unsigned int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7cdcbe7)
#10 0x0000000007cfea73 handleAssignment((anonymous namespace)::EvalInfo&, clang::Expr const*, (anonymous namespace)::LValue const&, clang::QualType, clang::APValue&) ExprConstant.cpp:0:0
#11 0x0000000007d4070e clang::StmtVisitorBase::Visit(clang::Stmt const*) ExprConstant.cpp:0:0
#12 0x0000000007d40fdd EvaluateLValue(clang::Expr const*, (anonymous namespace)::LValue&, (anonymous namespace)::EvalInfo&, bool) ExprConstant.cpp:0:0
#13 0x0000000007d0a8b6 Evaluate(clang::APValue&, (anonymous namespace)::EvalInfo&, clang::Expr const*) ExprConstant.cpp:0:0
#14 0x0000000007d134b4 EvaluateIgnoredValue((anonymous namespace)::EvalInfo&, clang::Expr const*) ExprConstant.cpp:0:0
#15 0x0000000007d24e97 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#16 0x0000000007d25648 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#17 0x0000000007d8eb2a EvaluateLoopBody((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) ExprConstant.cpp:0:0
#18 0x0000000007d253be EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#19 0x0000000007d25648 EvaluateStmt((anonymous namespace)::StmtResult&, (anonymous namespace)::EvalInfo&, clang::Stmt const*, clang::SwitchCase const*) (.part.0) ExprConstant.cpp:0:0
#20 0x0000000007d2b6fb HandleFunctionCall(clang::SourceLocation, clang::FunctionDecl const*, (anonymous namespace)::LValue const*, clang::Expr const*, llvm::ArrayRef, (anonymous namespace)::CallRef, clang::Stmt const*, (anonymous namespace)::EvalInfo&, clang::APValue&, (anonymous namespace)::LValue const*) ExprConstant.cpp:0:0
#21 0x0000000007d444c4 clang::Expr::isPotentialConstantExpr(clang::FunctionDecl const*, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x7d444c4)
#22 0x0000000006e7ff7e clang::Sema::CheckConstexprFunctionDefinition(clang::FunctionDecl const*, clang::Sema::CheckConstexprKind) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e7ff7e)
#23 0x0000000006e0578b clang::Sema::ActOnFinishFunctionBody(clang::Decl*, clang::Stmt*, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e0578b)
#24 0x0000000006b238c4 clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6b238c4)
#25 0x0000000006a297eb clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a297eb)
#26 0x0000000006a5fcad clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::Parser::ParsedTemplateInfo&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a5fcad)
#27 0x0000000006a1c94b clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a1c94b)
#28 0x0000000006a1d13f clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a1d13f)
#29 0x0000000006a24f6a clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a24f6a)
#30 0x0000000006a25f05 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr&, clang::Sema::ModuleImportState&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a25f05)
#31 0x0000000006a06f8a clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6a06f8a)
#32 0x0000000004c4cfb8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c4cfb8)
#33 0x0000000004f438b5 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f438b5)
#34 0x0000000004ec3b4e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ec3b4e)
#35 0x000000000503dc3d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x503dc3d)
#36 0x0000000000de5a6c cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde5a6c)
#37 0x0000000000ddc4ba ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#38 0x0000000000ddc63d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#39 0x0000000004cbf909 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#40 0x000000000422e2d4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x422e2d4)
#41 0x0000000004cbff1f clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#42 0x0000000004c80a02 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c80a02)
#43 0x0000000004c819ae clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c819ae)
#44 0x0000000004c88e05 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4c88e05)
#45 0x0000000000de1e61 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xde1e61)
#46 0x0000000000c8ceb4 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc8ceb4)
#47 0x0000727ea3a29d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#48 0x0000727ea3a29e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#49 0x0000000000ddbf55 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xddbf55)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

Reproduce the assertion with the godbolt.org example or the supplied constexpr function. Start at clang::APValue::getVectorElt in APValue.h and handleAssignment in ExprConstant.cpp, following the stack trace through constexpr evaluation. Done means this reproducer no longer aborts with the APValue assertion.

Written by the indexing model from the issue text.