[C++11] Use 'nullptr'. Serialization edition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209392 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209394 91177308-0d34-0410-b5e6-96231b3b80d8