[sil] Element shadowing of SILInstruction::getKind() by renaming MarkUninitializedInst::get{,MarkUninitialized}Kind().

Interestingly this problem can only occur if one invokes
MarkUninitializedInst::getKind() directly. Once our instruction is just a
SILInstruction, we call the appropriate method so we didn't notice it.

I used Xcode's refactoring functionality to find all of the invocation
locations.

lib/SILOptimizer/Transforms/AllocBoxToStack.cpp

@@ -518,7 +518,7 @@ static bool rewriteAllocBoxAsAllocStack(AllocBoxInst *ABI) {
     auto *User = HeapBox->getSingleUse()->getUser();
     if (auto *MUI = dyn_cast<MarkUninitializedInst>(User)) {
       HeapBox = MUI;
-      Kind = MUI->getKind();
+      Kind = MUI->getMarkUninitializedKind();
     }
   }