GlobalOpt: Don't assume we see all accesses to a global if it is imported from another module.

Currently we don't import (= deserialize) from other modules but we will if we serialize the StdlibUnitTests module.
I didn't add an explicit test case, because it's not that easy. But this change will be tested when running the suite with serialized StdlibUnitTest.

Author: eeckstein

lib/SILPasses/IPO/GlobalOpt.cpp

@@ -713,6 +713,11 @@ SILGlobalVariable *SILGlobalOpt::getVariableOfGlobalInit(SILFunction *AddrF) {
 }
 
 static bool canBeChangedExternally(SILGlobalVariable *SILG) {
+
+  // Don't assume anything about globals which are imported from other modules.
+  if (isAvailableExternally(SILG->getLinkage()))
+    return true;
+
   // Use access specifiers from the declarations,
   // if possible.
   if (auto *Decl = SILG->getDecl()) {