Fix crash when clang tries to build NSNumber literal after forward declaration

Bug report: http://llvm.org/bugs/show_bug.cgi?id=22561

Clang tries to create ObjCBoxedExpression of type 'NSNumber'
when 'NSNumber' has only forward declaration, this cause a crash later, 
when 'Sema' refers to a nil QualType of the whole expression.
Please, refer to the bug report for the better explanation.

llvm-svn: 229402