unique_ptr-ify ASTReaderListener in the ASTReader

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215317 91177308-0d34-0410-b5e6-96231b3b80d8
unique_ptr-ify FileSystemStatCache::takeNextStatCache

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215318 91177308-0d34-0410-b5e6-96231b3b80d8
unique_ptr-ify FrontendAction::takeCurrentASTUnit

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215319 91177308-0d34-0410-b5e6-96231b3b80d8