[NFC] Including <string> in llvm-cxxdump/Error.cpp

A [[ https://reviews.llvm.org/rGf6fa95b77f33c3690e4201e505cb8dce1433abd9 | recent commit  ]] removed `<string>` from `ErrorHandling.h`. The removal caused `<string>` to be no longer included for `llvm/tools/llvm-cxxdump/Error.cpp` which uses the string type.

This patch adds `<string>` to `llvm/tools/llvm-cxxdump/Error.cpp`.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D111354