[yaml2obj] - Allow setting custom sh_info for RawContentSection sections.

This is for tweaking SHT_SYMTAB sections.
Their sh_info contains the (number of symbols + 1) usually.
But for creating invalid inputs for test cases it would be convenient
to allow explicitly override this field from YAML.

Differential revision: https://reviews.llvm.org/D58779

llvm-svn: 355193