When LLDB reports progress on importing Swift modules, it was delivering
non-specific progress reports with only the title of "Importing Swift
modules" and no details. To report progress on activity within Swift,
LLDB first creates a progress report then updates that progress report
using a callback that LLDB sets and the Swift compiler invokes when
performing a full import.

When LLDB triggers Swift to import modules that were already imported before,
Swift will not perform a full import. Since the progress report would've
already been displayed, but the callback is never invoked, this leads
to the non-specific messages that were being displayed when importing
Swift modules.

This commit sets a unique pointer to create a progress report from
within the callback function instead of creating the report before
setting the callback. It also clears the callback on scope exit instead
of setting it to a new progress report.

Updates the test for Swift progress reporting by checking that the
"Importing Swift modules" message only appears twice on its own as the
rest of the reports for this category will have details attached.