[clang] Fix a -Wcast-qual GCC warning. NFC.

This fixes the following warning:

../tools/clang/lib/AST/Interp/Disasm.cpp: In member function ‘void clang::interp::Function::dump(llvm::raw_ostream&) const’:
../tools/clang/lib/AST/Interp/Disasm.cpp:43:25: warning: cast from type ‘const clang::interp::Function*’ to type ‘void*’ casts away qualifiers [-Wcast-qual]
   43 |     OS << " " << (void*)this << ":\n";
      |                         ^~~~