Preserve attributes on rewriting

[john-h-kastner]

3C ignored attributes when rewriting, so they would often be deleted or mess up rewriting in a way that made the uncompilable. Fixes #459 and adds a regression test for #450.

For example

void foo( const char *fmt, __builtin_ms_va_list argp);
__attribute__((ms_abi)) void bar ( const char *fmt, ...)  {
  __builtin_ms_va_list argp;
  __builtin_ms_va_start(argp, fmt);
  foo(fmt, argp);
  __builtin_ms_va_end(argp);
}

previously converted to

void foo( const char *fmt, __builtin_ms_va_list argp);
const char *fmt : itype(_Ptr<const char>), ...) {
  __builtin_ms_va_list argp;
  __builtin_ms_va_start(argp, fmt);
  foo(fmt, argp);
  __builtin_ms_va_end(argp);
}

but is now converted correctly.

---

benchmark run

[kyleheadley]

LGTM

[john-h-kastner]

Now that we have compilation checks for our benchmarks in the works, I'll hold off on merging this until we can check that it doesn't break any currently compiling benchmarks.