[json] no completions for enum values in alternatives

[kieferrm]

Testing #12600.

• VSCode Version: 1.6.0 insiders
• OS Version: MacOs

Steps to Reproduce:

1. Create the following launch.json file:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Launch",
            "type": "node"
        }
    ]
}

2. in the line 6 place caret in anywhere between the quotes in "node".

3. Ctrl+Space
   -> The only proposal you see is node. There should also be node2.

4. delete the word node in line 6

5. Ctrl+Space
   -> You get all valid proposal for launch config types.

[isidorn]

This is coming from the json schema suggest provider -> forwarding to @aeschli
Debug is properly contributing all the proposals as seen in step 5) of Kai's steps

[aeschli]

That's a subtlety of the JSON language server implementation. In the case of alternatives for the same object (in this case a configuration for each debug adapter) code assist looks which alternative matches. As type matches 100%, it only considers the schema for that cnfiguration and ignores all others. Thats useful for all properties except for the 'type' itself...
But it's tricky and I'd rather live with the bug for now