[ASTGen]: assert that position is within buffer bounds

Author: saehejkang

lib/ASTGen/Sources/ASTGen/Bridge.swift

@@ -34,6 +34,7 @@ extension BridgedSourceLoc {
   ) {
     if let start = buffer.baseAddress,
       position.utf8Offset >= 0 && position.utf8Offset < buffer.count {
+      assert(position.utf8Offset >= 0 && position.utf8Offset < buffer.count)
       self = SourceLoc_advanced(BridgedSourceLoc(raw: start), SwiftInt(position.utf8Offset))
     } else {
       self = nil