[asan_symbolize] Teach `asan_symbolize.py` to symbolicate partially symbolicated ASan reports.

Summary:
The use case here is to be able symbolicate ASan reports that might be
partially symbolicated, in particular where the function name is known but no source
location is available. This can be caused by missing debug info. Previously we
would only try to symbolicate completely unsymbolicated reports.

The code currently contains an unfortunate quirk to handle a darwin
specific bug (rdar://problem/49784442) in the way partially symbolicated
reports are emitted when the source location is missing.

rdar://problem/49476995

Reviewers: kubamracek, yln, samsonov, dvyukov, vitalybuka

Subscribers: aprantl, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D60533

llvm-svn: 363639