Optimize Host::GetThreadName() to read from /proc/$TID per Matt's suggestion.

CR: mkopec



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@187542 91177308-0d34-0410-b5e6-96231b3b80d8
Fix thread name updating in Linux. "thread list" should report correct names always now.
Created new LinuxThread class inherited from POSIXThread and removed linux / freebsd ifdefs
Removed several un-needed set thread name calls

CR (and multiple suggestions): mkopec



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@187545 91177308-0d34-0410-b5e6-96231b3b80d8