CMakeProjectManager: Fix clang code model when CMake PCHs are used

CMake gives the path to the cmake_pch.h[xx] file as relative path
to build directory. Making it absolute fixes the code model.

Fixes: QTCREATORBUG-22888
Change-Id: I2fdc080be779f22b737788be2074254290994aaa
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/libs/utils/fileutils.h

@@ -97,6 +97,7 @@ public:
 
     bool isChildOf(const FilePath &s) const;
     bool isChildOf(const QDir &dir) const;
+    bool startsWith(const QString &s) const;
     bool endsWith(const QString &s) const;
 
     bool isLocal() const;