Make the Expression Execution result enum available to the SB API layer.
Add a callback that will allow an expression to be cancelled between the
expression evaluation stages (for the ClangUserExpressions.)

<rdar://problem/16790467>, <rdar://problem/16573440>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@207944 91177308-0d34-0410-b5e6-96231b3b80d8