Increase initial offset into virtual file

Otherwise the PresumedLoc's suddely get valid once we avoid a
completely empty soure buffer in the next commit.

lib/Interpreter/IncrementalParser.h

@@ -70,8 +70,9 @@ namespace cling {
     // file ID of the memory buffer
     clang::FileID m_VirtualFileID;
 
-    // The next available unique sourcelocation offset.
-    unsigned m_VirtualFileLocOffset = 1; // skip the system sloc 0.
+    // The next available unique sourcelocation offset. Skip the system sloc 0
+    // and any offset that may actually exist in the virtual file.
+    unsigned m_VirtualFileLocOffset = 100;
 
     // CI owns it
     DeclCollector* m_Consumer;