SpecialCaseList: Add support for parsing multiple input files.

Summary:
This change allows users to create SpecialCaseList objects from
multiple local files. This is needed to implement a proper support
for -fsanitize-blacklist flag (allow users to specify multiple blacklists,
in addition to default blacklist, see PR22431).

DFSan can also benefit from this change, as DFSan instrumentation pass now
accepts ABI-lists both from -fsanitize-blacklist= and -mllvm -dfsan-abilist flags.

Go bindings are fixed accordingly.

Test Plan: regression test suite

Reviewers: pcc

Subscribers: llvm-commits, axw, kcc

Differential Revision: http://reviews.llvm.org/D7367

llvm-svn: 228155