[global-variable] When deserializing global variables, only deserialize if the current module does not have a gv with the same name.

This is testing by deserialization not blowing up.

<rdar://problem/18562242>

Swift SVN r22555

lib/Serialization/DeserializeSIL.cpp

@@ -333,6 +333,18 @@ SILFunction *SILDeserializer::getFuncForReference(StringRef name) {
   return readSILFunction(*iter, nullptr, name, /*declarationOnly*/ true);
 }
 
+/// Helper function to find a SILGlobalVariable given its name. It first checks
+/// in the module. If we can not find it in the module, we attempt to
+/// deserialize it.
+SILGlobalVariable *SILDeserializer::getGlobalForReference(StringRef name) {
+  // Check to see if we have a global by this name already.
+  if (SILGlobalVariable *g = SILMod.lookUpGlobalVariable(name))
+    return g;
+
+  // Otherwise, look for a global with this name in the module.
+  return readGlobalVar(name);
+}
+
 /// Deserialize a SILFunction if it is not already deserialized. The input
 /// SILFunction can either be an empty declaration or null. If it is an empty
 /// declaration, we fill in the contents. If the input SILFunction is
@@ -918,7 +930,7 @@ bool SILDeserializer::readSILInstruction(SILFunction *Fn, SILBasicBlock *BB,
     Identifier Name = MF->getIdentifier(ValID);
 
     // Find the global variable.
-    SILGlobalVariable *g = readGlobalVar(Name.str());
+    SILGlobalVariable *g = getGlobalForReference(Name.str());
     assert(g && "Can't deserialize global variable");
     assert(g->getLoweredType().getAddressType() ==
            getSILType(Ty, (SILValueCategory)TyCategory) &&

lib/Serialization/DeserializeSIL.h

@@ -93,6 +93,7 @@ namespace swift {
     SILFunction *getFuncForReference(StringRef Name);
     SILFunction *lookupSILFunction(StringRef Name);
     SILVTable *readVTable(serialization::DeclID);
+    SILGlobalVariable *getGlobalForReference(StringRef Name);
     SILGlobalVariable *readGlobalVar(StringRef Name);
     SILWitnessTable *readWitnessTable(serialization::DeclID,
                                       SILWitnessTable *existingWt,