crash when const attribute is applied to variadic snprintf

Reproducer:
https://godbolt.org/z/jKeoqrE46
```cpp
int __attribute__((const)) snprintf(char *, __SIZE_TYPE__, const char *, ...);

long foo(char c, long d) { return snprintf(&c, d, ""); }
```

Backtrace:
```console
Attributes 'readnone and writeonly' are incompatible!
ptr @snprintf
Attributes 'readnone and readonly' are incompatible!
ptr @snprintf
fatal error: error in backend: Broken module found, compilation aborted!
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: /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 -O1
1. parser at end of file
2. Code generation
#0 0x0000000004423f28 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4423f28)
#1 0x0000000004420dd4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4420dd4)
#2 0x000000000442140c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x442140c)
#3 0x00000000043608c6 llvm::CrashRecoveryContext::HandleExit(int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43608c6)
#4 0x000000000441897e llvm::sys::Process::Exit(int, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x441897e)
#5 0x0000000000da8c10 LLVMErrorHandler(void*, char const*, bool) cc1_main.cpp:0:0
#6 0x000000000436a343 llvm::report_fatal_error(llvm::Twine const&, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x436a343)
#7 0x000000000436a4a1 (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x436a4a1)
#8 0x0000000003dbb17d (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3dbb17d)
#9 0x0000000003cf6aed llvm::FPPassManager::doFinalization(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3cf6aed)
#10 0x0000000003d03a61 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3d03a61)
#11 0x0000000004841f58 (anonymous namespace)::EmitAssemblyHelper::TimeCodegenPasses(llvm::function_ref) (.isra.0) BackendUtil.cpp:0:0
#12 0x0000000004850fab (anonymous namespace)::EmitAssemblyHelper::emitAssembly(clang::BackendAction, std::unique_ptr>, clang::BackendConsumer*) BackendUtil.cpp:0:0
#13 0x0000000004851493 clang::emitBackendOutput(clang::CompilerInstance&, clang::CodeGenOptions&, llvm::StringRef, llvm::Module*, clang::BackendAction, llvm::IntrusiveRefCntPtr, std::unique_ptr>, clang::BackendConsumer*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4851493)
#14 0x0000000004f24abc clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f24abc)
#15 0x0000000006d316bd clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6d316bd)
#16 0x0000000004f24ee8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f24ee8)
#17 0x0000000005252b28 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5252b28)
#18 0x00000000051df6ad clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x51df6ad)
#19 0x0000000005327dbd clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5327dbd)
#20 0x0000000000dab3bc cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdab3bc)
#21 0x0000000000da1fda ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#22 0x0000000000da215d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#23 0x0000000004f9ce59 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#24 0x0000000004360804 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4360804)
#25 0x0000000004f9d9c5 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#26 0x0000000004f558a2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f558a2)
#27 0x0000000004f5a207 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f5a207)
#28 0x0000000004f64775 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f64775)
#29 0x0000000000da78ba clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda78ba)
#30 0x0000000000cc35ba main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcc35ba)
#31 0x000071807122a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#32 0x000071807122a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#33 0x0000000000da1985 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xda1985)
clang++: error: clang frontend command failed with exit code 70 (use -v to see invocation)
Compiler returned: 70
```

Start by running the linked Godbolt reproducer with Clang at -O1 and inspect the generated IR or backend diagnostics around the conflicting attributes. Trace the code-generation path that handles the const variadic snprintf declaration; done means the reproducer compiles without the broken-module crash and the regression is covered by an appropriate compiler test.

Written by the indexing model from the issue text.