[NFC][IR] Make Module::getGlobalList() private

This patch adds several missing GlobalList modifier functions, like
removeGlobalVariable(), eraseGlobalVariable() and insertGlobalVariable().
There is no longer need to access the list directly so it also makes
getGlobalList() private.

Differential Revision: https://reviews.llvm.org/D144027

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -3693,7 +3693,7 @@ Error BitcodeReader::globalCleanup() {
       UpgradedVariables.emplace_back(&GV, Upgraded);
   for (auto &Pair : UpgradedVariables) {
     Pair.first->eraseFromParent();
-    TheModule->getGlobalList().push_back(Pair.second);
+    TheModule->insertGlobalVariable(Pair.second);
   }
 
   // Force deallocation of memory for these vectors to favor the client that