[LibFuzzer] Provide stub implementation of __sanitizer_cov_trace_pc_indir

[delcypher]

Summary

[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.

This combined with http://reviews.llvm.org/D20943 is enough to fix compilation of the LibFuzzer tests on OSX.

[kcc]

Add something like "currently not used " to the comment.

llvm/trunk/lib/Fuzzer/FuzzerTracePC.cpp

static void HandlePC(uint32_t PC) {
   // We take 12 bits of PC and mix it with the previous PCs.
   uintptr_t Next = (Prev << 5) ^ (PC & 4095);
   CurrentMap.Update(Next);
   Prev = Next;
 }
 
 } // namespace fuzzer
 
-extern "C" void __sanitizer_cov_trace_pc() {
+extern "C" {
+void __sanitizer_cov_trace_pc() {
   fuzzer::HandlePC(static_cast<uint32_t>(
       reinterpret_cast<uintptr_t>(__builtin_return_address(0))));
 }
+
+void __sanitizer_cov_trace_pc_indir(int *) {
+  // Stub to allow linking with code built with
+  // -fsanitize=indirect-calls,trace-pc.
+  // This isn't used currently.
+}
+}