[lldb] Make sure we don't drop asynchronous output when sourcing files

Summary:
If a command from a sourced file produces asynchronous output, this
output often does not make its way to the user. This happens because the
asynchronous output machinery relies on the iohandler stack to ensure
the output does not interfere with the things the iohandler is doing.

However, if this happens near the end of the command stream then by the
time the asynchronous output is produced we may already have already
started tearing down the sourcing session. Specifically, we may already
pop the relevant iohandler, leaving the stack empty.

This patch makes sure this kind of output gets printed by adding a
fallback to IOHandlerStack::PrintAsync to print the output directly if
the stack is empty. This is safe because if we have no iohandlers then
there is nothing to synchronize.

Reviewers: JDevlieghere, clayborg

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D75454