[ObjectYAML] - Redefine LLVM_YAML_IS_SEQUENCE_VECTOR(llvm::yaml::Hex*) as LLVM_YAML_I...

[ObjectYAML] - Redefine LLVM_YAML_IS_SEQUENCE_VECTOR(llvm::yaml::Hex*) as LLVM_YAML_IS_FLOW_SEQUENCE_VECTOR.

I am using it in https://reviews.llvm.org/D69399.

This change changes how obj2yaml dumps arrays of `llvm::yaml::Hex8/llvm::yaml::Hex16/llvm::yaml::Hex32`
from:

```
PayloadBytes:
- 0x01
- 0x02
...
```

To

```
PayloadBytes:    [ 0x01, 0x02, ... ]
```

The latter way is shorter and looks better for arrays.

Differential revision: https://reviews.llvm.org/D69558