CppTools: Distinguish between ProjectFile::{unclassified,unsupported}

There was no way to determine whether ProjectFile::classify() was run or
not. Now, ProjectFile::classify() returns ProjectFile::Unsupported
instead of ProjectFile::Unclassified.

Change-Id: I660d0e42044bdefcac38058c6f4a3425983a6d93
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppprojectfile.h

@@ -36,6 +36,7 @@ class CPPTOOLS_EXPORT ProjectFile
 public:
     enum Kind {
         Unclassified,
+        Unsupported,
         AmbiguousHeader,
         CHeader,
         CSource,