Clang: Fix completion after qualification (::)

...and maybe other cases.

Since

    Clang: fix findStartOfName handling
    commit 82d0650b11

the proposal's base position was calculated wrong. As a result, an early
return triggert in CodeAssistantPrivate::displayProposal (call to
newProposal->hasItemsToPropose(prefix, reason)) and no completions were
displayed.

Fix by ensuring that the added code from the mentioned commit is only
called when needed, namely only for function expressions.

Task-number: QTCREATORBUG-19083
Change-Id: I8f23c9b7186f9d81159939c8b3ef475a09bbe760
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

tests/unit/unittest/activationsequencecontextprocessor-test.cpp

@@ -147,7 +147,9 @@ TEST(ActivationSequenceContextProcessor, TemplateFunctionLeftParen)
 TEST(ActivationSequenceContextProcessor, TemplateFunctionSecondParameter)
 {
     ClangCompletionAssistInterface interface("foo<X>(", 7);
-    int startOfname = ContextProcessor::findStartOfName(&interface, 6);
+    int startOfname = ContextProcessor::findStartOfName(&interface,
+                                                        6,
+                                                        ContextProcessor::NameCategory::Function);
 
     ASSERT_THAT(startOfname, 0);
 }