Use Frame list context when GetThreadContext is unavailable

Use Frame list context when GetThreadContext is unavailable

When DAC tries to obtain thread context for debugee thread on the system without working GetThreadContext, it uses filter context
and if it is NULL, it goes through thread Frame list looking for a Frame that can provide meaningful CONTEXT to start the stackwalk.
Filter context can be NULL if the thread is currently running native code, or manage exception just has been thrown (first chance).
Before this change DBI stackwalker would fail to provide stack trace in such cases.