[PATCH] Don't emit structors for available_externally globals (PR19933)

[Reid Kleckner]

lgtm

================
Comment at: lib/CodeGen/AsmPrinter/AsmPrinter.cpp:1354
@@ +1353,3 @@
+      if (GV->hasAvailableExternallyLinkage())
+        // Don't dynamically initialize available_externally vars.
+        continue;
----------------
This should briefly explain why, with something like:
// If the associated variable is available_externally, some other TU will provide its dynamic initializer.

http://reviews.llvm.org/D4022