Replace Sema::CheckVariableInitList, CheckConstantInitList, and CheckForCharArrayInitializer with CheckInitializerListTypes. The new routine now supports struct/union initializers.

This is a modified version of a patch by Eli Friedman. This version passes all of the clang tests and supports vectors:-)

Note that Eli and I are still unsatisfied the implementation of CheckInitializerListTypes(). We'd like to find a more elegant implementation. Nevertheless, this is a complex part of the standard (and may be hard to simplify). Since the complexity is localized to this routine, we can iterate without too much disruption:-)

llvm-svn: 46339