This commit is similar to 788e3b31,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

See commit 788e3b31 for an explanation.

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

An ino_t is usually a 64 bit integer, and some file systems (such as
Linux's kernel NFS client) really uses the upper 32 bit.  This can
lead to false positives in the directory loop detection in
FindAncestorLoop().  Increasing these two attributes (in
StorageFileInfo and Directory) to 64 bit adds little overhead, but
makes the check a lot safer.

This code (added 7 years ago with commit b233c145) has been obsoleted
by the SongLoader class (added 3 years ago).

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

If the base class is not accessible, the "catching" the base class
won't work.  This caused the fatal error:

 terminate called after throwing an instance of 'LibmpdclientError'