Fix warnings generated by clang-cl.

There were a couple of real bugs here regarding error checking and
signed/unsigned comparisons, but mostly these were just noise.

There was one class of bugs fixed here which is particularly
annoying, dealing with MSVC's non-standard behavior regarding
the underlying type of enums.  See the comment in
lldb-enumerations.h for details.  In short, from now on please use
FLAGS_ENUM and FLAGS_ANONYMOUS_ENUM when defining enums which
contain values larger than can fit into a signed integer.

llvm-svn: 233943