[Clang] Fix handling of non-member functions in isNormalAssignmentOperator() (#115880)

This patch correctes the handling of non-member functions in the
`isNormalAssignmentOperator` function within `CheckExprLifetime.cpp`.

The previous implementation incorrectly assumed that `FunctionDecl` is
always a `CXXMethodDecl`, leading to potential null pointer
dereferencing.

Change: - Correctly handle the case where `FD` is not a `CXXMethodDecl`
by using `FD->getParamDecl(0)->getType()`.

This fix ensures that the function correctly handles non-member
assignment operators, such as:

`struct S {}; void operator|=(S, S) {}`

This change improves the robustness of the `isNormalAssignmentOperator`
function by correctly identifying and handling different types of
function declarations.

Author: smanna12

clang/lib/Sema/CheckExprLifetime.cpp

@@ -490,7 +490,7 @@ static bool isNormalAssignmentOperator(const FunctionDecl *FD) {
       if (MD && MD->isCXXInstanceMember())
         LHST = Ctx.getLValueReferenceType(MD->getFunctionObjectParameterType());
       else
-        LHST = MD->getParamDecl(0)->getType();
+        LHST = FD->getParamDecl(0)->getType();
       if (Ctx.hasSameType(RetT, LHST))
         return true;
     }