[llvm] [YAML][NFC] precommit wrong test case (PR #131782)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

I was about to report the same as one of the bots had reported here, this breaks compilation with GCC:
```
llvm-project/llvm/unittests/Support/YAMLIOTest.cpp:1280:20: error: explicit specialization of ‘template<class T> struct llvm::yaml::MappingTrait’ outside its namespace must use a nested-name-specifier [-fpermissive]
 1280 | template <> struct MappingTraits<V> {
      |                    ^~~~~~~~~~~~~~~~
```

I'll push a revert soon unless someone else beats me to it.

https://github.com/llvm/llvm-project/pull/131782