llvm-symbolizer: Fix FRAME handling of missing AT_name.

Summary:
llvm-symbolizer protocol is empty string means end-of-output.
Do not emit empty string when a function or a variable do not have a
name for any reason. Emit "??".

Reviewers: pcc, vitalybuka, jdoerfert

Subscribers: srhines, hiraditya, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70626