ObjC: Handle boolean fixed type for enum.

Before this commit, we assert failure in ImplicitCastExpr
"unheralded conversion to bool". This commit fixes the assertion by using
the correct cast type when the fixed type is boolean.

This commit also fixes the behavior for Microsoft mode as well, since
Obj-C and Microsoft mode share the same code path.

rdar://24999533

llvm-svn: 264167