[win32] Suppress win32 swiftshader warnings (#562)

Suppresses several Windows-specific warnings emitted during swiftshader
builds:

* On swiftshader builds against Win32 C++ headers, warnings are emitted
  about subclassing std::iterator, which was deprecated in C++17. This
  affects a header file in swiftshader's third-party directory:
  third_party/swiftshader/third_party/llvm-subzero/include/llvm/ADT/iterator.h

* Three functions in libVulkan.cpp are re-declared with a difference in
  dllexport attribute only.

* Silence implicit string conversion warnings in two places in
  swiftshader where asserts of the form assert(!"Error msg") are used.

Issue: https://github.com/flutter/flutter/issues/59199