Pretend that enum constants have enum type when inferring a block return type.

In C, enum constants have the type of the enum's underlying integer type,
rather than the type of the enum. (This is not true in C++.) This leads to
odd warnings when returning enum constants directly in blocks with inferred
return types. The easiest way out of this is to pretend that, like C++, enum
constants have enum type when being returned from a block.

<rdar://problem/11662489>

llvm-svn: 158899