Add DidStartExecuting/WillFinishExecuting methods to Expression.

These methods can be used by the derived expression types to perform expression
specific and/or language specific actions before and after the expression runs.
(ThreadPlanCallUserExpression is modified to call these methods on the
expression immediately before/after execution of the expression).

The immediate motivation is allowing Swift expressions to notify the swift
runtime that exclusivity enforcement should be suspended while the expression
runs (we want LLDB expressions to be able to access variables even when they're
considered exclusively owned by someone else in the original program).

Reviewed in https://reviews.llvm.org/D32889

llvm-svn: 302314