AutoTest: Cache PCH lookups

Avoid slowing down scans of bigger projects by caching
lookups of paths inside precompiled headers.

Change-Id: I17652ba36ab761cc06ecce151e41253271236881
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/autotest/itestparser.h

@@ -95,6 +95,7 @@ public:
     static bool precompiledHeaderContains(const CPlusPlus::Snapshot &snapshot,
                                           const Utils::FilePath &filePath,
                                           const QRegularExpression &headerFileRegex);
+
 protected:
     CPlusPlus::Snapshot m_cppSnapshot;
     CppTools::WorkingCopy m_workingCopy;