Frontend: Propagate ASTReaderListener API in ChainedASTReaderListener

ASTReaderListener's documentation states that visitInputFile will be
called based on the return values of needsInputFileVisitation and
needsSystemInputFileVisitation, but ChainedASTReaderListener may call
these methods on a child listener based on the values returned by the
other child.

Even worse, the calls to visitInputFile may be short-circuited due to
the use of the boolean or, so the calls to visit may not occur at all
for the second listener.

This updates ChainedASTReaderListener::visitInputFile to propagate the
ASTReaderListener behaviour to both children.