Add a warning for direct-list-initialization of a variable with a deduced type

Add a warning for direct-list-initialization of a variable with a deduced type
(or of a lambda init-capture, which is sort-of such a variable). The semantics
of such constructs will change when we implement N3922, so we intend to warn on
this in Clang 3.6 then change the semantics in Clang 3.7.

llvm-svn: 228792