[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 `lld-x86_64-ubuntu-fast` running on `as-builder-4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/33/builds/20652

<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 :: DebugInfo/AMDGPU/dwarfdump-rel.yaml' FAILED ********************
Exit Code: 134

Command Output (stderr):
--
/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp # RUN: at line 1
+ /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp
yaml2obj: /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/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/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp
 #0 0x000055f4a299f5c0 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj+0x1c85c0)
 #1 0x000055f4a299c43f llvm::sys::RunSignalHandlers() (/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj+0x1c543f)
 #2 0x000055f4a299c592 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #3 0x00007fd1309c2520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007fd130a169fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007fd1309c2476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007fd1309a87f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x00007fd1309a871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x00007fd1309b9e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x000055f4a2872410 llvm::yaml::MappingTraits<llvm::ELFYAML::FileHeader>::mapping(llvm::yaml::IO&, llvm::ELFYAML::FileHeader&) (/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj+0x9b410)
#10 0x000055f4a289af74 llvm::yaml::MappingTraits<llvm::ELFYAML::Object>::mapping(llvm::yaml::IO&, llvm::ELFYAML::Object&) (/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj+0xc3f74)
#11 0x000055f4a2814109 llvm::yaml::convertYAML(llvm::yaml::Input&, llvm::raw_ostream&, llvm::function_ref<void (llvm::Twine const&)>, unsigned int, unsigned long) (/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj+0x3d109)
#12 0x000055f4a27f0b69 main (/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj+0x19b69)
#13 0x00007fd1309a9d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#14 0x00007fd1309a9e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#15 0x000055f4a280f315 _start (/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj+0x38315)
/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.script: line 2: 3479850 Aborted                 (core dumped) /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/bin/yaml2obj /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp

--

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


```

</details>

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