Refactor out the circular reference to LambdaExpr in CXXRecordDecl.

A prior commit of this patch was reverted because it was within the blamelist's purview of a failing test.  The failure of that test has been addressed here: http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20131021/091546.html.  Therefore I am recommitting this patch (all tests pass on windows, except for the usual modules & index suspects that never pass on my box).

Some background: Both Doug and Richard had asked me in Chicago to remove the circular reference in CXXRecordDecl to LambdaExpr by factoring out and storing the needed information from LambdaExpr directly into CXXRecordDecl.
 
In addition, I have added an IsGenericLambda flag - this makes life a little easier when we implement capturing, and are Sema-analyzing the body of a lambda (and the calloperator hasn't been wired to the closure class yet). Any inner lambdas can have potential captures that could require walking up the scope chain and checking if any generic lambdas are capture-ready. This 'bit' makes some of that checking easier. 

No change in functionality.

This patch was approved by Doug with minor modifications (comments were cleaned up, and all data members were converted from bool/enum to unsigned, as requested): 
http://llvm-reviews.chandlerc.com/D1856

Thanks!



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193246 91177308-0d34-0410-b5e6-96231b3b80d8
A decl never becomes unused. Make that explicit in the API.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193248 91177308-0d34-0410-b5e6-96231b3b80d8