Clang: Optimize file path cache

We now fetch all directories and sources from the database at file
path cache creation.

Change-Id: I92510b49a234128f4c82b840611db82ead3f1a54
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/unittest/pchcreator-test.cpp

@@ -114,8 +114,9 @@ protected:
     NiceMock<MockPchManagerClient> mockPchManagerClient;
     NiceMock<MockClangPathWatcher> mockClangPathWatcher;
     NiceMock<MockBuildDependenciesStorage> mockBuildDependenciesStorage;
+    ClangBackEnd::FilePathCaching filePathCache{database};
     ClangBackEnd::PchCreator creator{environment,
-                                     database,
+                                     filePathCache,
                                      mockPchManagerClient,
                                      mockClangPathWatcher,
                                      mockBuildDependenciesStorage};