[DebugInfo][NFC] Add a comment on the ordering of DILineInfo frames.

Add a comment to getFrame() to mention that frames are stored in
bottom-up, i.e. leaf to root in order of increasing index.

Differential Revision: https://reviews.llvm.org/D122033

llvm/include/llvm/DebugInfo/DIContext.h

@@ -90,6 +90,8 @@ class DIInliningInfo {
 public:
   DIInliningInfo() = default;
 
+  /// Returns the frame at `Index`. Frames are stored in bottom-up
+  /// (leaf-to-root) order with increasing index.
   const DILineInfo &getFrame(unsigned Index) const {
     assert(Index < Frames.size());
     return Frames[Index];