[demangler] Buffer peeking needs buffer

The output buffer has a 'back' member, which returns NUL when you try
it with an empty buffer.  But there are no use cases that need that
additional functionality.  This makes the 'back' member behave more
like STL containers' back members.  (It still returns a value, not a
reference.)

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D123201