[lib/Support/YAMLTraits] - Don't print leading zeroes when dumping Hex8/Hex16/Hex32 types.

When we produce an YAML output, we also print leading zeroes currently.
An output might look like this:

```
- Name:    .dynsym
  Type:    SHT_DYNSYM
  Address: 0x0000000000001000
  EntSize: 0x0000000000000018
```

There are probably no reason to print leading zeroes.
It just makes harder to read values. This patch stops printing them.
The output becomes like:

```
- Name:    .dynsym
  Type:    SHT_DYNSYM
  Address: 0x1000
  EntSize: 0x18
```

This affects obj2yaml mostly, but also dsymutil and llvm-xray tools output.

Differential revision: https://reviews.llvm.org/D90930