[X86] crash on __attribute__((nvptx_kernel))

Related issue: https://github.com/llvm/llvm-project/issues/157028

Reproducer:
https://godbolt.org/z/1xf6caqPG
```cpp
__attribute__((nvptx_kernel)) void foo(int *ret) {
*ret = 1;
}
```

Backtrace:
```console
Unknown kernel calling convention
UNREACHABLE executed at /root/llvm-project/llvm/tools/clang/lib/CodeGen/TargetInfo.cpp:129!
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. :1:36: LLVM IR generation of declaration 'foo'
3. :1:36: Generating code for declaration 'foo'
#0 0x00000000042387e8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x42387e8)
#1 0x0000000004235c14 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4235c14)
#2 0x0000000004179e68 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#3 0x00007a0d09442520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
#4 0x00007a0d094969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
#5 0x00007a0d09442476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
#6 0x00007a0d094287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
#7 0x0000000004185a4a (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4185a4a)
#8 0x00000000049a44e0 clang::CodeGen::TargetCodeGenInfo::createEnqueuedBlockKernel(clang::CodeGen::CodeGenFunction&, llvm::Function*, llvm::Type*) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x49a44e0)
#9 0x0000000004a7b6a0 clang::CodeGen::CodeGenTypes::arrangeLLVMFunctionInfo(clang::CanQual, clang::CodeGen::FnInfoOpts, llvm::ArrayRef>, clang::FunctionType::ExtInfo, llvm::ArrayRef, clang::CodeGen::RequiredArgs) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4a7b6a0)
#10 0x0000000004a7d21e arrangeLLVMFunctionInfo(clang::CodeGen::CodeGenTypes&, bool, llvm::SmallVectorImpl>&, clang::CanQual) CGCall.cpp:0:0
#11 0x0000000004a80e1e clang::CodeGen::CodeGenTypes::arrangeFunctionDeclaration(clang::GlobalDecl) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4a80e1e)
#12 0x000000000481cf16 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x481cf16)
#13 0x0000000004817d34 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4817d34)
#14 0x0000000004818bd0 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4818bd0)
#15 0x00000000048235b3 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#16 0x0000000004b87f29 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#17 0x0000000004b77044 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b77044)
#18 0x000000000692f354 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x692f354)
#19 0x0000000004b83928 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4b83928)
#20 0x0000000004e71515 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4e71515)
#21 0x0000000004df231e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4df231e)
#22 0x0000000004f69ced clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4f69ced)
#23 0x0000000000dd0670 cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdd0670)
#24 0x0000000000dc71da ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#25 0x0000000000dc735d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#26 0x0000000004bed1c9 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#27 0x000000000417a304 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x417a304)
#28 0x0000000004bed7df clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#29 0x0000000004bae572 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4bae572)
#30 0x0000000004baf51e clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4baf51e)
#31 0x0000000004bb6c55 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4bb6c55)
#32 0x0000000000dccb99 clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdccb99)
#33 0x0000000000c7bb64 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc7bb64)
#34 0x00007a0d09429d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#35 0x00007a0d09429e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#36 0x0000000000dc6c75 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xdc6c75)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134
```

Start with the Godbolt reproducer and inspect clang/lib/CodeGen/TargetInfo.cpp at the reported assertion, especially createEnqueuedBlockKernel and arrangeLLVMFunctionInfo in the backtrace. Determine how the nvptx_kernel attribute reaches the X86 code-generation path; done means compiling the reproducer no longer aborts at the unreachable assertion.

Written by the indexing model from the issue text.