Fix build break: clang no longer supports -ast-dump-xml.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192155 91177308-0d34-0410-b5e6-96231b3b80d8
Clarify how to handle a thread register context with a gap
in the middle because of alignment rules.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@192159 91177308-0d34-0410-b5e6-96231b3b80d8