[flang] Allow user to define free via BIND(C) (#78428)

A user defining and using free/malloc via BIND(C) would previously cause
flang to crash when generating LLVM IR with error "redefinition of
symbol named 'free'". This was caused by flang codegen not expecting to
find a mlir::func::FuncOp definition of these function and emitting a
new mlir::LLVM::FuncOp that later conflicted when translating the
mlir::func::FuncOp.