warn on missing blacklist, better errmsg

[ramosian-glider]

Originally reported on Google Code with ID 14

clang -mllvm -asan-blacklist=asan_blacklist.ignore
currently fails on a missing asan_blacklist.ignore file.

I improved the error message.

And I prefer a warning over the fatal exit as I have multiple blacklist 
files in relative paths, but only for certain subprojects.
Missing a blacklist file should just produce a warning IMHO.

patch attached.

Reported by reini.urban on 2011-11-30 16:12:27

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- _Attachment: [blacklist-warn.patch](https://storage.googleapis.com/google-code-attachments/address-sanitizer/issue-14/comment-0/blacklist-warn.patch)_

[ramosian-glider]

More informative warning is ok, I'll commit it with some other changes. 
Warning instead of error is not ok, imho. 

Reported by konstantin.s.serebryany on 2011-12-01 06:08:48

• Status changed: Accepted

[ramosian-glider]

Just landed the change (llvm r145611) that calls report_fatal_error when the blacklist
is not found. 

Reported by konstantin.s.serebryany on 2011-12-01 18:59:12

• Status changed: Fixed

[ramosian-glider]

Adding Project:AddressSanitizer as part of GitHub migration.

Reported by ramosian.glider on 2015-07-30 09:12:58

• Labels added: ProjectAddressSanitizer