Add a new SBExecutionContext class that wraps an ExecutionContextRef. This class is a convenient way at the API level to package a target,process,thread and frame all together - or just a subset of those

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218808 91177308-0d34-0410-b5e6-96231b3b80d8