* Remove PPCallbacks::FileNotFound() override

Since according to [1] it had been broken anyway.

  [1]: llvm/llvm-project@7a124f4

* Include clang directories with -isystem/SYSTEM

This is to suppress errors like:

    In file included from /workdir/woboq_codebrowser/generator/commenthandler.cpp:27:
    In file included from /usr/lib/llvm-15/include/clang/AST/CommentParser.h:16:
    /usr/lib/llvm-15/include/clang/AST/Comment.h:149:54: warning: arithmetic between different enumeration types ('clang::comments::Comment::(unnamed enum at /usr/lib/llvm-15/include/clang/AST/Comment.h:66:3)' and 'clang::comments::CommandInfo::(unnamed enum at /usr/lib/llvm-15/include/clang/AST/CommentCommandTraits.h:43:3)') is deprecated [-Wdeprecated-anon-enum-enum-conversion]
      enum { NumBlockCommandCommentBits = NumCommentBits +
                                          ~~~~~~~~~~~~~~ ^

Refs: https://s3.amazonaws.com/clickhouse-test-reports/41046/f1df5fb537b97fa6e67db121995d72ecdf797cf7/push_to_dockerhub_aarch64.html

* Support llvm-15 for PreprocessorCallback::InclusionDirective()

Refs: llvm/llvm-project@d79ad2f1dbc2d