[Clang] Fix an assertion failure introduced by #93430 (#100313)

Summary:
The PR #93430 introduced an assertion that did not make any sense. and
caused a regression. The fix is to simply remove the assertion.

No changelog. the intent is to backport this fix to clang 19.

Test Plan: 

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Differential Revision: https://phabricator.intern.facebook.com/D60250628

clang/lib/Sema/SemaExpr.cpp

@@ -5730,7 +5730,6 @@ static bool isParenthetizedAndQualifiedAddressOfExpr(Expr *Fn) {
   if (!UO || UO->getOpcode() != clang::UO_AddrOf)
     return false;
   if (auto *DRE = dyn_cast<DeclRefExpr>(UO->getSubExpr()->IgnoreParens())) {
-    assert(isa<FunctionDecl>(DRE->getDecl()) && "expected a function");
     return DRE->hasQualifier();
   }
   if (auto *OVL = dyn_cast<OverloadExpr>(UO->getSubExpr()->IgnoreParens()))

clang/test/SemaCXX/cxx2b-deducing-this.cpp

@@ -895,6 +895,10 @@ void g() {
 }
 
 namespace P2797 {
+
+int bar(void) { return 55; }
+int (&fref)(void) = bar;
+
 struct C {
   void c(this const C&);    // #first
   void c() &;               // #second
@@ -915,6 +919,8 @@ struct C {
     (&C::c)(C{});
     (&C::c)(*this);     // expected-error {{call to non-static member function without an object argument}}
     (&C::c)();
+
+    (&fref)();
   }
 };
 }