Fix broken NativeProcessLinux.cpp after signature change of ResolveExecutable.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222184 91177308-0d34-0410-b5e6-96231b3b80d8
Change HostThread::GetNativeThread() to return a derived reference.

Previously using HostThread::GetNativeThread() required an ugly
cast to most-derived type.  This solves the issue by simply returning
the derived type directly.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@222185 91177308-0d34-0410-b5e6-96231b3b80d8