[Global Opt] add SILFunction *InitializerF to SILGlobalVariable.

It is defaulted to nullptr. When it is set, we increment the SILFunction's
ref count to keep it alive.

It will be used in followon patches for globals that can be statically
initialized.


Swift SVN r21983

lib/SIL/SILModule.cpp

@@ -96,6 +96,11 @@ SILModule::SILModule(Module *SwiftModule, const DeclContext *associatedDC)
 }
 
 SILModule::~SILModule() {
+  // Decrement ref count for each SILGlobalVariable with static initializers.
+  for (SILGlobalVariable &v : silGlobals)
+    if (v.getInitializer())
+      v.getInitializer()->decrementRefCount();
+
   for (SILFunction *F : InlinedFunctions)
     F->decrementRefCount();