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

Currently we have the `Flags` property that allows to
set flags for a section. The problem is that it does not
allow us to set an arbitrary value, because of bit fields
validation under the hood. An arbitrary values can be used
to test specific broken cases.

We probably do not want to relax the validation, so this
patch adds a `ShSize` property that allows to
override the `sh_size`. It is inline with others `Sh*` properties
we have already.

Differential revision: https://reviews.llvm.org/D71411