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/cppeditor/baseeditordocumentprocessor.cpp

@@ -22,7 +22,7 @@ namespace CppEditor {
 */
 
 BaseEditorDocumentProcessor::BaseEditorDocumentProcessor(QTextDocument *textDocument,
-                                                         const QString &filePath)
+                                                         const Utils::FilePath &filePath)
     : m_filePath(filePath),
       m_textDocument(textDocument)
 {