Support: Avoid errors with LLVM_FALLTHROUGH in clang 3.6 and below in C mode

Older versions of clang defined __has_cpp_attribute in C mode, but
would choke on scoped attributes, as per llvm.org/PR23435. Since we
support building with clang all the way back to 3.1, we have to work
around this issue.

llvm-svn: 280326