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/libs/cplusplus/pp-engine.h

@@ -36,6 +36,8 @@
 
 #include <functional>
 
+namespace Utils { class FilePath; }
+
 namespace CPlusPlus {
 
 class Environment;
@@ -57,6 +59,8 @@ public:
 public:
     Preprocessor(Client *client, Environment *env);
 
+    QByteArray run(const Utils::FilePath &filePath, const QByteArray &source,
+                   bool noLines = false, bool markGeneratedTokens = true);
     QByteArray run(const QString &filename, const QByteArray &source,
                    bool noLines = false, bool markGeneratedTokens = true);