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/cppcodestylesettingspage.cpp

@@ -45,7 +45,8 @@ static void applyRefactorings(QTextDocument *textDocument, TextEditorWidget *edi
     Environment env;
     Preprocessor preprocess(nullptr, &env);
     const QByteArray preprocessedSource
-        = preprocess.run(QLatin1String("<no-file>"), textDocument->toPlainText().toUtf8());
+        = preprocess.run(Utils::FilePath::fromParts({}, {}, u"<no-file>"),
+                         textDocument->toPlainText().toUtf8());
 
     Document::Ptr cppDocument = Document::create(QLatin1String("<no-file>"));
     cppDocument->setUtf8Source(preprocessedSource);