[yaml2obj] - Add a way to override sh_type section field.

This adds the `ShType` key similar to others `Sh*` keys we have.

My use case is the following. Imagine we have a `SHT_SYMTAB_SHNDX`
section and want to hide it from a dumper. The natural way would be to
do something like:

```
  - Name:    .symtab_shndx
    Type:    [[TYPE=SHT_SYMTAB_SHNDX]]
    Entries: [ 0, 1 ]

```

and then change the TYPE from `SHT_SYMTAB_SHNDX` to something else,
for example to `SHT_PROGBITS`.

But we have a problem: regular sections does not have `Entries` key,
so yaml2obj will be unable to produce a section.

The solution is to introduce a `ShType` key to override the final type.

This is not the first time I am facing the need to change the type. I
was able to invent workarounds or solved issues differently in the past,
but finally came to conclusion that we just should support the `ShType`.

Differential revision: https://reviews.llvm.org/D84738