Autogen: Do not treat hard-coded -I/usr/include exclusion as implicit include
cmLocalGenerator::GetIncludeDirectoriesImplicit method is called by
cmQtAutoGenInitializer to get the compiler's list of implicit include directories. Since !2957 (merged) the method hard-codes exclusion of
/usr/include for historical reasons. However, it should not be reported as a real implicit include directory unless the compiler really has it.
Refactor the logic to distinguish the hard-coded exclusion of
/usr/include from the real list of implicit include directories.
Fixes: #19100 (closed)