CppTools: Fix using updated project part

1. Open a project consisting of two subprojects referencing the same
   source file.
2. Open the source file.
3. Click '#' in the editor toolbar and select the second project (part).
4. Update the project file, e.g. add a define
   ==> Editor does not reflect the added define

This is due to comparing project part pointers. Fix by using the project
part id that remains stable across project manager updates.

Change-Id: Ifd1a113e55ebe2ecf036cd7caafdbfd6e4cdf415
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppprojectpartchooser.h

@@ -51,7 +51,7 @@ public:
     ProjectPartInfo choose(
             const QString &filePath,
             const ProjectPartInfo &currentProjectPartInfo,
-            const ProjectPart::Ptr &manuallySetProjectPart,
+            const QString &preferredProjectPartId,
             const ProjectExplorer::Project *activeProject,
             Language languagePreference,
             bool projectHasChanged) const;