[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 `ml-opt-rel-x86-64` running on `ml-opt-rel-x86-64-b1` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/185/builds/22524

<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):
--
/b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj /b/ml-opt-rel-x86-64-b1/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /b/ml-opt-rel-x86-64-b1/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp # RUN: at line 1
+ /b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj /b/ml-opt-rel-x86-64-b1/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /b/ml-opt-rel-x86-64-b1/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp
yaml2obj: /b/ml-opt-rel-x86-64-b1/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: /b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj /b/ml-opt-rel-x86-64-b1/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /b/ml-opt-rel-x86-64-b1/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp
 #0 0x000055b9b8ffab78 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj+0x1c5b78)
 #1 0x000055b9b8ff7d05 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #2 0x00007f8a0aef5140 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x13140)
 #3 0x00007f8a0a9f6d61 raise (/lib/x86_64-linux-gnu/libc.so.6+0x38d61)
 #4 0x00007f8a0a9e0537 abort (/lib/x86_64-linux-gnu/libc.so.6+0x22537)
 #5 0x00007f8a0a9e040f (/lib/x86_64-linux-gnu/libc.so.6+0x2240f)
 #6 0x00007f8a0a9ef6e2 (/lib/x86_64-linux-gnu/libc.so.6+0x316e2)
 #7 0x000055b9b8ed1ad0 llvm::yaml::MappingTraits<llvm::ELFYAML::FileHeader>::mapping(llvm::yaml::IO&, llvm::ELFYAML::FileHeader&) (/b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj+0x9cad0)
 #8 0x000055b9b8efdc1c llvm::yaml::MappingTraits<llvm::ELFYAML::Object>::mapping(llvm::yaml::IO&, llvm::ELFYAML::Object&) (/b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj+0xc8c1c)
 #9 0x000055b9b8e72447 llvm::yaml::convertYAML(llvm::yaml::Input&, llvm::raw_ostream&, llvm::function_ref<void (llvm::Twine const&)>, unsigned int, unsigned long) (/b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj+0x3d447)
#10 0x000055b9b8e4ec0a main (/b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj+0x19c0a)
#11 0x00007f8a0a9e1d7a __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x23d7a)
#12 0x000055b9b8e6d87a _start (/b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj+0x3887a)
/b/ml-opt-rel-x86-64-b1/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.script: line 2: 3629863 Aborted                 /b/ml-opt-rel-x86-64-b1/build/bin/yaml2obj /b/ml-opt-rel-x86-64-b1/llvm-project/llvm/test/DebugInfo/AMDGPU/dwarfdump-rel.yaml -o /b/ml-opt-rel-x86-64-b1/build/test/DebugInfo/AMDGPU/Output/dwarfdump-rel.yaml.tmp

--

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


```

</details>

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