[tsan] Do not instrument accesses to the gcov counters array

There is a known intended race here. This is a follow-up to r264805,
which disabled tsan instrumentation for updates to instrprof counters.
For more background on this please see the discussion in D18164.

llvm-svn: 273202

llvm/lib/Transforms/Instrumentation/ThreadSanitizer.cpp

@@ -271,6 +271,10 @@ static bool shouldInstrumentReadWriteFromAddress(Value *Addr) {
             /*AddSegment=*/false)))
         return false;
     }
+
+    // Check if the global is in the GCOV counters array.
+    if (GV->getName() == "__llvm_gcov_ctr")
+      return false;
   }
   return true;
 }