Build ANCM with /LTCG?

While building the solution locally, I noticed the following notice was printed a few times when ANCM is compiled:

```console
libnethost.lib(trace.cpp.obj) : MSIL .netmodule or module compiled with /GL found; restarting link with /LTCG; add /LTCG to the link command line to improve linker performance
```

[Example](https://dev.azure.com/dnceng-public/public/_build/results?buildId=716589&view=logs&j=1b89928a-2219-5ef9-602f-f95beb3da4dc&t=31281680-1367-5e42-8b2d-0ecd34ac2092&l=322)

I don't really know where the right place to add such a change would be (or indeed even if it's appropriate), but I couldn't find any other note of this anywhere so thought I'd raise an issue for it as seems on face value it would be a useful change if it made the build faster, especially if it's a one-liner to add into a project file somewhere to add the flag.