[LibFuzzer] Provide stub implementation of __sanitizer_cov_trace_pc_indir

Calls to this function are currently injected by the
``SanitizerCoverageModule`` pass when the both the ``indirect-calls``
and ``trace-pc`` sanitizer coverage options are enabled and the code
being instrumented has indirect calls. Previously because LibFuzzer did
not define this function this would lead to link errors when building
some of the tests on OSX.

Differential Revision: http://reviews.llvm.org/D20946

llvm-svn: 271938