[PATCH] [asancov] Write coverage directly to a memory-mapped file

[Kostya Serebryany]

================
Comment at: lib/sanitizer_common/sanitizer_coverage.cc:120
@@ +119,3 @@
+
+void CoverageData::Extend(uptr npcs) {
+  if (!pc_fd) return;
----------------
this function deserves a comment.

http://reviews.llvm.org/D3874