The `/GR` flag has been on by default since MSVC cl 14.0 from VS 2005.
Remove it from the default flags to make it easier for projects to pass
`/GR-` themselves to turn it off.

Projects may be using string processing to replace `/GR` with another
flag, so we cannot simply drop it.  Add a policy to drop it in a
compatible way.

Fixes: #21428