Use MapVector::pop_back() per LLVM r175538.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175539 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 901eeb1..bc212bc 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -6998,7 +6998,7 @@
       // FIXME: std::move
       IdentifierInfo *II = PendingIdentifierInfos.back().first;
       SmallVector<uint32_t, 4> DeclIDs = PendingIdentifierInfos.back().second;
-      PendingIdentifierInfos.erase(II);
+      PendingIdentifierInfos.pop_back();
 
       SetGloballyVisibleDecls(II, DeclIDs, &TopLevelDecls[II]);
     }