Fix Clang-tidy modernize-use-nullptr and modernize-use-default warnings in source/Commands/CommandObjectThread.cpp; other minor fixes.
Fix Clang-tidy modernize-use-nullptr and modernize-use-default warnings in source/Commands/CommandObjectThread.cpp; other minor fixes.


git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@261936 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb