Teach __builtin_unpredictable to work through implicit casts.

The __builtin_unpredictable implementation is confused by any implicit
casts, which happen in C++.  This patch strips those off so that
if/switch statements now work with it in C++.

Change-Id: I73c3bf4f1775cd906703880944f4fcdc29fffb0a
llvm-svn: 348969