[ASTGen]: fix force-unwrap for buffer.baseAddress

Author: saehejkang

lib/ASTGen/Sources/ASTGen/Bridge.swift

@@ -30,10 +30,10 @@ extension BridgedSourceLoc {
   /// Form a source location at the given absolute position in `buffer`.
   init(
     at position: AbsolutePosition,
-    in buffer: UnsafeBufferPointer<UInt8>?
+    in buffer: UnsafeBufferPointer<UInt8>
   ) {
-    precondition(position.utf8Offset >= 0 && position.utf8Offset < buffer!.count)
-    self = SourceLoc_advanced(BridgedSourceLoc(raw: buffer!.baseAddress), SwiftInt(position.utf8Offset))
+    precondition(position.utf8Offset >= 0 && position.utf8Offset < buffer.count)
+    self = SourceLoc_advanced(BridgedSourceLoc(raw: buffer.baseAddress!), SwiftInt(position.utf8Offset))
   }
 }