Replace Sema::CheckInitializerListTypes() with a helper class (InitListChecker) that...

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@50544 91177308-0d34-0410-b5e6-96231b3b80d8