CPlusPlus: Proliferate FilePath use

The starts with CppDocument::filePath(), plus a bit of the fallout

This is one patch of potentially many. It is hard to draw the
line where to stop this kind of chunk, this here converts a few
additional functions for which including it in the patch looked
like less churn than without.

Converting is mostly fromString/toString, with a few exceptions
for "already seem" like caches, that use cheaper "path()" to
avoid likely performance regressions (on Windows FilePath
comparison is currently case-insenstive, and more expensive).

There should be no difference for local operation with this patch.

Change-Id: I7b35f98a0a6f0bfed4ea0f8f987faf586f7a8f2b
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/classview/classviewmanager.cpp

@@ -275,7 +275,7 @@ void Manager::initialize()
         if (doc.data() == nullptr)
             return;
 
-        d->m_awaitingDocuments.insert(FilePath::fromString(doc->fileName()));
+        d->m_awaitingDocuments.insert(doc->filePath());
         d->m_timer.start(400); // Accumulate multiple requests into one, restarts the timer
     });