Modify ASTReaderListener to allow visiting the input files of an AST file.

We can pass such an input-file-visiting ASTReaderListener to ASTReader::readASTFileControlBlock.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181238 91177308-0d34-0410-b5e6-96231b3b80d8