Replace Sema::CheckInitializerListTypes() with a helper class (InitListChecker) that synthesizes implicit InitListExpr's when appropriate (see InitListExpr comments in Expr.h for more details). I also moved the code to SemaInit.cpp, to help reduce clutter in SemaDecl.cpp.

NOTE: This work is incomplete and still fails many tests (as a result, it isn't enabled yet). Nevertheless, I wanted to check it in so I can work on it from home.
llvm-svn: 50544