Make elf::ScriptConfig a LinkerScript class member variable.

LinkerScript used to be a template class, so we couldn't instantiate
that class in elf::link. We instantiated ScriptConfig class earlier
instead so that the linker script parser can store configurations to
the object.

Now that LinkerScript is not a template, it doesn't make sense to
separate ScriptConfig from LinkerScript. This patch merges them.

llvm-svn: 298457