[YAML] - Teach yaml2obj/obj2yaml to work with numeric relocation values.

That may be useful if we want to produce or parse object containing
broken relocation values using yaml2obj/obj2yaml.

Previously that was impossible because only enum values were parsed
correctly, this patch allows to put any numeric value as a
relocation type.

Differential revision: https://reviews.llvm.org/D34758

llvm-svn: 306814