[ELF] - Refactor of tls_index implementation for tls local dynamic model.

Patch contains the next 2 changes:
1) static variable Out<ELFT>::LocalModuleTlsIndexOffset moved to Out<ELFT>::Got. At fact there is no meaning for it to be separated from GOT class because at each place of using it anyways needs to call GOT`s getVA(). Also it is impossible to have that offset and not have GOT.
2) addLocalModuleTlsIndex -> addLocalModelTlsIndex (word "Module" changed to "Model"). Not sure was it a mistype or not but I think that update is closer to Urlich terminology.

Differential revision: http://reviews.llvm.org/D15113

llvm-svn: 254428