FuncUnwinders: prefer debug_frame over eh_frame

The two sections usually contain the same information, and we rarely
have both kinds of entries for a single function. However, in theory the
debug_frame plan can be more complete, whereas eh_frame is only required
to be correct at places where exceptions can be thrown.

Reviewers: jasonmolenda, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D62374

llvm-svn: 361758