Support for overloading built-in operators

Hi! While Polygeist seems to have good support for structs, member functions and templating, I came across issues when trying to overload built-in operators.

A simple case is:

struct Pair {
  int first;
  int second;
  Pair operator+(Pair other) {
    return Pair{this->first + other.first, this->second + other.second};
  }
};
Pair timesTwo(Pair p) { return p + p; }

running: mlir-clang -v -S -function=* -scal-rep=0 --memref-fullrank ${file} produces:

CXXConstructExpr 0xeb86c20 'struct Pair' 'void (struct Pair &&) noexcept' elidable
`-MaterializeTemporaryExpr 0xeb86c08 'struct Pair' xvalue
  `-CXXOperatorCallExpr 0xeb86bd0 'struct Pair' '+'
    |-ImplicitCastExpr 0xeb86bb8 'struct Pair (*)(struct Pair)' <FunctionToPointerDecay>
    | `-DeclRefExpr 0xeb86b70 'struct Pair (struct Pair)' lvalue CXXMethod 0xeb682e8 'operator+' 'struct Pair (struct Pair)'
    |-DeclRefExpr 0xeb86980 'struct Pair' lvalue ParmVar 0xeb86800 'p' 'struct Pair'
    `-CXXConstructExpr 0xeb86b40 'struct Pair' 'void (const struct Pair &) noexcept'
      `-ImplicitCastExpr 0xeb869c0 'const struct Pair' lvalue <NoOp>
        `-DeclRefExpr 0xeb869a0 'struct Pair' lvalue ParmVar 0xeb86800 'p' 'struct Pair'
cleanup not handled
CXXConstructExpr 0xeb86780 'struct Pair' 'void (struct Pair &&) noexcept' elidable
`-MaterializeTemporaryExpr 0xeb68be8 'struct Pair' xvalue
  `-CXXFunctionalCastExpr 0xeb68758 'struct Pair' functional cast to struct Pair <NoOp>
    `-InitListExpr 0xeb685c8 'struct Pair'
      |-BinaryOperator 0xeb68478 'int' '+'
      | |-ImplicitCastExpr 0xeb68448 'int' <LValueToRValue>
      | | `-MemberExpr 0xeb683c8 'int' lvalue ->first 0xeb68130
      | |   `-CXXThisExpr 0xeb683b8 'struct Pair *' this
      | `-ImplicitCastExpr 0xeb68460 'int' <LValueToRValue>
      |   `-MemberExpr 0xeb68418 'int' lvalue .first 0xeb68130
      |     `-DeclRefExpr 0xeb683f8 'struct Pair' lvalue ParmVar 0xeb681f8 'other' 'struct Pair'
      `-BinaryOperator 0xeb68558 'int' '+'
        |-ImplicitCastExpr 0xeb68528 'int' <LValueToRValue>
        | `-MemberExpr 0xeb684a8 'int' lvalue ->second 0xeb68198
        |   `-CXXThisExpr 0xeb68498 'struct Pair *' this
        `-ImplicitCastExpr 0xeb68540 'int' <LValueToRValue>
          `-MemberExpr 0xeb684f8 'int' lvalue .second 0xeb68198
            `-DeclRefExpr 0xeb684d8 'struct Pair' lvalue ParmVar 0xeb681f8 'other' 'struct Pair'
cleanup not handled

Start by running the provided Pair example with the listed mlir-clang command and inspect handling of CXXOperatorCallExpr, CXXConstructExpr, and the reported “cleanup not handled” cases. Done means the overloaded built-in operator example is accepted without those unsupported-node errors and produces the expected MLIR output.