Calculate proper common type of lambdas

[dlangBugzillaToGithub]

Kenji Hara (@9rnsr) reported this on 2014-04-21T04:08:24Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=12605

Description

Currently this code:

pragma(msg, typeof(true ? (x => x) : ((int x) => x * 2)));

prints 'void'. But it could print 'int function(int)' by fitting parameter types.

More cases:

pragma(msg, typeof(true ? (()=>1) : (()=>1.0)));
// fitting return types - could be 'double function()'

pragma(msg, typeof(true ? ((int x) => 1) : (delegate(int x)=>1)));
// fitting context type --> could be 'int delegate(int)'