[NativeProcessLinux] Reduce the number of casts

Summary:
NPL used to be peppered with casts of the NativeThreadProtocol objects into NativeThreadLinux. I
move these closer to the source where we obtain these objects. This way, the rest of the code can
assume we are working with the correct type of objects.

Reviewers: ovyalov, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12187

llvm-svn: 245681