Add Lazy<T>(T value) constructor

[ghost]

Fix dotnet/corefx#2578

[ghost]

@AlexGhiondea, @justinvp, please take a look and let me know where to add tests for this addition. :)

[justinvp]

model.xml needs to be updated with the new constructor.

After that, the following will need to happen:

1. Porting the change to Lazy.cs in the CoreRT repo.
2. Updating the contract (ref assembly) for System.Runtime in the CoreFX repo and incrementing the version as appropriate. I'm not sure how versioning of System.Runtime is being handled (i.e. when we're able to add new APIs to this contract), but this may have to wait until the new Lazy constructor is added to a future version of the desktop framework.

[ghost]

@justinvp, thanks. I have updated model.xml and opened a PR in CoreRT repo dotnet/corert#1538.

[jkotas]

@AlexGhiondea is offline till September.
cc @danmosemsft @weshaggard

Looks like this was approved in dotnet/corert#1538 and it is pretty straightforward. LGTM.

This will need to a special care on TFS side, so whoever merges it needs to be prepared to take care of it. (I cannot take care of it at the moment.)

[danmoseley]

If the question for me was versioning of the corefx contract - I don't know when that will happen as it's not clear when the larger API work will get into master. This change LGTM though.

[danmoseley]

(And such versioning can happen w/o the larger APi work of course.)