TextEditor: Allow incremental proposals

... and make use of them with clangd.
This way, users can get immediate feedback when a new proposal entry
has been found, rather than having to wait until all of them have been
collected.

Change-Id: I2adfe0153aa7a058f28eb3bd65c71dd30ea018e0
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/test/clangautomationutils.cpp

@@ -61,8 +61,9 @@ public:
         bool gotResults = false;
 
         processor->setAsyncCompletionAvailableHandler(
-                    [this, &gotResults] (TextEditor::IAssistProposal *proposal) {
+                    [this, processor, &gotResults] (TextEditor::IAssistProposal *proposal) {
             QTC_ASSERT(proposal, return);
+            QTC_CHECK(!processor->running());
             proposalModel = proposal->model();
             delete proposal;
             gotResults = true;