[Sanitizer] Teach ReadFileToBuffer to distinguish empty file from inaccessible file.

Summary:
This fixes https://code.google.com/p/address-sanitizer/issues/detail?id=399
(sanitizers crash with empty suppression files).

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11284

llvm-svn: 242594