[flang][NFC] Address reported "possible missing return" (#86523)

A function uses "if constexpr" to consider all possible types in a
variant, but looks as if it can fall out without returning an
expression. Add a final "else" with a crash to make things more clear
and to protect against unlikely future extensions of the type.

Fixes #86391.

Author: klausler

flang/lib/Semantics/tools.cpp

@@ -1287,6 +1287,8 @@ static bool StopAtComponentPre(const Symbol &component) {
     return !IsPointer(component);
   } else if constexpr (componentKind == ComponentKind::PotentialAndPointer) {
     return true;
+  } else {
+    DIE("unexpected ComponentKind");
   }
 }