C++: Fix 'already seen files' when indexing

Resetting the environment (after each *.cpp file) did not clear the
already seen files (m_included). Because of that the succeeding header
files were not parsed correctly - the environments of the mistakenly
already seen header files were not merged in.

Note that this change slow downs the parsing/indexing of files to its
original speed, as it was before the problematic commit (and it is in
2.7):

    commit 82e347095c
    C++: Untangle include file resolving from loading.

Task-number: QTCREATORBUG-9205
Change-Id: Iea57b7c59ea04a3c8843fd1291f4c375382958fc
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cpppreprocessor.cpp

@@ -138,6 +138,7 @@ void CppPreprocessor::resetEnvironment()
 {
     m_env.reset();
     m_processed.clear();
+    m_included.clear();
 }
 
 void CppPreprocessor::getFileContents(const QString &absoluteFilePath,