Disable -Wimplicit-fallthrough when not using C++.

The rationale is that there is no good workflow to silence the warning
for specific cases, other than using pragmas.  This is because the
attribute to decorate an explicit fall through is only available
in C++11.

By that argument, this should probably also be disabled unless one
is using C++11, but apparently there is an explicit test case for
this warning when using C++98.  This will require further discussion
on cfe-commits.

Fixes:  <rdar://problem/12584746>
llvm-svn: 167655