[Sema] Issue a warning for integer overflow in struct initializer

Clang wasn't issuing a warning when compiling the following code:

struct s {

unsigned x;

} s = {

.x = 4 * 1024 * 1024 * 1024

};

rdar://problem/23399683

Differential Revision: http://reviews.llvm.org/D15097

llvm-svn: 257357