Fix Issue 20687 - Allow member function address as const initializer

[BorisCarvajal]

It seems like an artificial restriction, seeing that you can use a manifest constant.

[dlang-bot]

Thanks for your pull request and interest in making D better, @BorisCarvajal! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20687	normal	Allow member function address as const initializer

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#10946"

[ibuclaw]

Do virtual functions still produce an error?

[BorisCarvajal]

@ibuclaw Nope

[ibuclaw]

Then there lies one possible problem. :-)

[BorisCarvajal]

With great power comes great responsibility. ;)