[ELF] - Step to combine LinkerScript and LinkerScriptBase

We can move all not templated functionality to LinkerScriptBase.
Patch do that for hasPhdrsCommands() and shows how it helps to detemplate
things in other places.

Probably we should be able to merge these 2 classes into single one after such steps.
Even if not, it still looks as reasonable cleanup for me.

Differential revision: https://reviews.llvm.org/D30895

llvm-svn: 297714