[clang] malformed __identifier directive causes preprocessor assertion failure with -fms-compatibility

Reproducer:
https://godbolt.org/z/xEYs9vb51
```cpp
# 1 __identifier(foo
```

Backtrace:
```console
clang++: /root/llvm-project/llvm/tools/clang/lib/Lex/PPDirectives.cpp:97: clang::SourceRange clang::Preprocessor::DiscardUntilEndOfDirective(clang::Token&, llvm::SmallVectorImpl*): Assertion `Tmp.isNot(tok::eof) && "EOF seen while discarding directive tokens"' failed.
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 -fms-compatibility
1. :1:3: current parser token '1'
#0 0x00000000044aca88 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44aca88)
#1 0x00000000044a9ce4 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44a9ce4)
#2 0x00000000044aa31c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44aa31c)
#3 0x00000000043e6898 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
#4 0x00007cd3fa045330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x00007cd3fa09ec0c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9ec0c)
#6 0x00007cd3fa04527e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007cd3fa0288ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x00007cd3fa02881b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
#9 0x00007cd3fa03b517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#10 0x0000000008868d95 clang::Preprocessor::DiscardUntilEndOfDirective(clang::Token&, llvm::SmallVectorImpl*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x8868d95)
#11 0x00000000088704c2 clang::Preprocessor::HandleDigitDirective(clang::Token&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x88704c2)
#12 0x000000000887f193 clang::Preprocessor::HandleDirective(clang::Token&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x887f193)
#13 0x000000000881488a clang::Lexer::LexTokenInternal(clang::Token&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x881488a)
#14 0x0000000008816afa clang::Lexer::Lex(clang::Token&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x8816afa)
#15 0x00000000088c382c clang::Preprocessor::Lex(clang::Token&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x88c382c)
#16 0x0000000006e10012 clang::Parser::ConsumeToken() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6e10012)
#17 0x0000000006dfbd61 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x6dfbd61)
#18 0x0000000004fc0788 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4fc0788)
#19 0x00000000052e0e28 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x52e0e28)
#20 0x000000000526e92d clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x526e92d)
#21 0x00000000053b564d clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x53b564d)
#22 0x0000000000d8f6fc cc1_main(llvm::ArrayRef, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8f6fc)
#23 0x0000000000d863fa ExecuteCC1Tool(llvm::SmallVectorImpl&, llvm::ToolContext const&, llvm::IntrusiveRefCntPtr) driver.cpp:0:0
#24 0x0000000000d8657d int llvm::function_ref&)>::callback_fn&)>(long, llvm::SmallVectorImpl&) driver.cpp:0:0
#25 0x0000000005034329 void llvm::function_ref::callback_fn>, std::__cxx11::basic_string, std::allocator>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#26 0x00000000043e6c94 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43e6c94)
#27 0x0000000005034e95 clang::driver::CC1Command::Execute(llvm::ArrayRef>, std::__cxx11::basic_string, std::allocator>*, bool*) const (.part.0) Job.cpp:0:0
#28 0x0000000004ff32c2 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ff32c2)
#29 0x0000000004ff7c27 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4ff7c27)
#30 0x0000000005002185 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x5002185)
#31 0x0000000000d8bbea clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd8bbea)
#32 0x0000000000cc977a main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xcc977a)
#33 0x00007cd3fa02a1ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#34 0x00007cd3fa02a28b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#35 0x0000000000d85da5 _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xd85da5)
clang++: error: clang frontend command failed due to signal (use -v to see invocation)
Program terminated with signal SIGABRT (6)
Compiler returned: 134
```

Start with llvm/tools/clang/lib/Lex/PPDirectives.cpp, especially Preprocessor::HandleDigitDirective and DiscardUntilEndOfDirective, using the reported reproducer with -fms-compatibility. Trace how the malformed __identifier directive reaches EOF, then verify the input produces diagnostics without triggering an assertion or crash.

Written by the indexing model from the issue text.