Fix a buffer-size bug when the first DW_OP_piece is undefined

and document the shortcomings of LLDB's partially defined DW_OP_piece
handling.

This would manifest as "DW_OP_piece for offset foo but top of stack is
of size bar".

rdar://problem/46262998

Differential Revision: https://reviews.llvm.org/D72880

(cherry picked from commit f55ab6f9)