[llvm] Allow "[[FLAGS=<none>]]" value in the ELF Fileheader Flags field (PR #143845)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-ubuntu-fast` running on `sie-linux-worker` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/30811

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: tools/llvm-readobj/ELF/file-header-machine-types.test' FAILED ********************
Exit Code: 134

Command Output (stderr):
--
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/tools/llvm-readobj/ELF/file-header-machine-types.test -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/test/tools/llvm-readobj/ELF/Output/file-header-machine-types.test.tmp.none.o -D MACHINE=EM_NONE # RUN: at line 3
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/tools/llvm-readobj/ELF/file-header-machine-types.test -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/test/tools/llvm-readobj/ELF/Output/file-header-machine-types.test.tmp.none.o -D MACHINE=EM_NONE
yaml2obj: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/include/llvm/Support/YAMLTraits.h:1680: void llvm::yaml::IO::processKeyWithDefault(const char*, std::optional<_Tp>&, const std::optional<_Tp>&, bool, Context&) [with T = llvm::ELFYAML::ELF_EF; Context = llvm::yaml::EmptyContext]: Assertion `!DefaultValue && "std::optional<T> shouldn't have a value!"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/tools/llvm-readobj/ELF/file-header-machine-types.test -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/test/tools/llvm-readobj/ELF/Output/file-header-machine-types.test.tmp.none.o -D MACHINE=EM_NONE
 #0 0x0000632355338d90 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj+0x1c8d90)
 #1 0x0000632355335c0f llvm::sys::RunSignalHandlers() (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj+0x1c5c0f)
 #2 0x0000632355335d62 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #3 0x00007b6ef6c0f520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007b6ef6c639fc __pthread_kill_implementation ./nptl/pthread_kill.c:44:76
 #5 0x00007b6ef6c639fc __pthread_kill_internal ./nptl/pthread_kill.c:78:10
 #6 0x00007b6ef6c639fc pthread_kill ./nptl/pthread_kill.c:89:10
 #7 0x00007b6ef6c0f476 gsignal ./signal/../sysdeps/posix/raise.c:27:6
 #8 0x00007b6ef6bf57f3 abort ./stdlib/abort.c:81:7
 #9 0x00007b6ef6bf571b _nl_load_domain ./intl/loadmsgcat.c:1177:9
#10 0x00007b6ef6c06e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
#11 0x000063235520bba0 llvm::yaml::MappingTraits<llvm::ELFYAML::FileHeader>::mapping(llvm::yaml::IO&, llvm::ELFYAML::FileHeader&) (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj+0x9bba0)
#12 0x0000632355234704 llvm::yaml::MappingTraits<llvm::ELFYAML::Object>::mapping(llvm::yaml::IO&, llvm::ELFYAML::Object&) (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj+0xc4704)
#13 0x00006323551ad899 llvm::yaml::convertYAML(llvm::yaml::Input&, llvm::raw_ostream&, llvm::function_ref<void (llvm::Twine const&)>, unsigned int, unsigned long) (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj+0x3d899)
#14 0x000063235518a2e9 main (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj+0x1a2e9)
#15 0x00007b6ef6bf6d90 __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#16 0x00007b6ef6bf6e40 call_init ./csu/../csu/libc-start.c:128:20
#17 0x00007b6ef6bf6e40 __libc_start_main ./csu/../csu/libc-start.c:379:5
#18 0x00006323551a8a95 _start (/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj+0x38a95)
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/test/tools/llvm-readobj/ELF/Output/file-header-machine-types.test.script: line 320: 487394 Aborted                 (core dumped) /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/yaml2obj /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/llvm/test/tools/llvm-readobj/ELF/file-header-machine-types.test -o /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/test/tools/llvm-readobj/ELF/Output/file-header-machine-types.test.tmp.none.o -D MACHINE=EM_NONE

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/143845