CompilationDatabase: Speed up parsing project file

Avoid expensive indexOf and check for the exactly same flags
before applying the full filter.

Change-Id: I6936b2022a2b439aad7bf0a65280c3db16d00c34
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/compilationdatabaseprojectmanager/compilationdatabaseutils.h

@@ -38,6 +38,8 @@ class Macro;
 
 namespace CompilationDatabaseProjectManager {
 
+QStringList filterFromFileName(const QStringList &flags, QString baseName);
+
 void filteredFlags(const QString &fileName,
                    const QString &workingDir,
                    QStringList &flags,