Switch case IDs conflict between chained PCHs; since there is no need to be global, make them local to a decl.

llvm-svn: 117540
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index f09873a..6497b78 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3284,6 +3284,9 @@
 /// source each time it is called, and is meant to be used via a
 /// LazyOffsetPtr (which is used by Decls for the body of functions, etc).
 Stmt *ASTReader::GetExternalDeclStmt(uint64_t Offset) {
+  // Switch case IDs are per Decl.
+  ClearSwitchCaseIDs();
+
   // Offset here is a global offset across the entire chain.
   for (unsigned I = 0, N = Chain.size(); I != N; ++I) {
     PerFileData &F = *Chain[N - I - 1];
@@ -4252,6 +4255,10 @@
   return SwitchCaseStmts[ID];
 }
 
+void ASTReader::ClearSwitchCaseIDs() {
+  SwitchCaseStmts.clear();
+}
+
 /// \brief Record that the given label statement has been
 /// deserialized and has the given ID.
 void ASTReader::RecordLabelStmt(LabelStmt *S, unsigned ID) {