Remove HostThreadLinux/Free/NetBSD

Summary:
These classes existed only because of the GetName() static function,
which can be moved to a more natural place anyway. I move the linux
version to NativeProcessLinux (and get rid of ProcFileReader), the
freebsd version to ProcessFreeBSD (and fix a bug where it was using the
current process ID, instead of the inferior pid), and remove the NetBSD
version (which was probably incorrect anyway, as it assumes the current
process instead of the inferior.

I also add an llgs test to that verifies thread names are read
correctly.

Reviewers: zturner, krytarowski, emaste

Subscribers: lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D30981

llvm-svn: 298058