Fix handling of initialization from parenthesized initializer list.

This change fixes a crash on initialization of a reference from ({}) during
template instantiation and incidentally improves diagnostics.

This reverts a prior attempt to handle this in r286721. Instead, we teach the
initialization code that initialization cannot be performed if a source type
is required and the initializer is an initializer list (which is not an
expression and does not have a type), and likewise for function-style cast
expressions.

llvm-svn: 298676