DebugInfo: Migrate DISubprogram::describes() to new hierarchy, NFC

I don't really like this function at all -- I think it should be as
simple as `return getFunction() == F` -- but for now this seems like the
best we can do.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234778 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DebugInfoMetadata.h

@@ -1386,6 +1386,11 @@ public:
   void replaceFunction(std::nullptr_t) { replaceOperandWith(7, nullptr); }
   /// @}
 
+  /// \brief Check if this subprogram decribes the given function.
+  ///
+  /// FIXME: Should this be looking through bitcasts?
+  bool describes(const Function *F) const;
+
   static bool classof(const Metadata *MD) {
     return MD->getMetadataID() == MDSubprogramKind;
   }