Revert "[lldb] Move redundant persistent variable counter to ClangPersistentVariables"

This reverts commit r367842 since it wasn't quite as NFC as advertised
and broke Swift support.  See https://reviews.llvm.org/D46083 for the
rationale behind the original functionality.

rdar://problem/54619322

llvm-svn: 370126
diff --git a/lldb/source/Expression/ExpressionVariable.cpp b/lldb/source/Expression/ExpressionVariable.cpp
index 61df940..ed8da0a 100644
--- a/lldb/source/Expression/ExpressionVariable.cpp
+++ b/lldb/source/Expression/ExpressionVariable.cpp
@@ -76,3 +76,13 @@
     }
   }
 }
+
+ConstString PersistentExpressionState::GetNextPersistentVariableName(
+    Target &target, llvm::StringRef Prefix) {
+  llvm::SmallString<64> name;
+  {
+    llvm::raw_svector_ostream os(name);
+    os << Prefix << target.GetNextPersistentVariableIndex();
+  }
+  return ConstString(name);
+}