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.cpp

@@ -925,6 +925,12 @@ bool FilePath::isChildOf(const QDir &dir) const
     return isChildOf(FilePath::fromString(dir.absolutePath()));
 }
 
+/// \returns whether FilePath startsWith \a s
+bool FilePath::startsWith(const QString &s) const
+{
+    return m_data.startsWith(s, HostOsInfo::fileNameCaseSensitivity());
+}
+
 /// \returns whether FilePath endsWith \a s
 bool FilePath::endsWith(const QString &s) const
 {