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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@280326 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit ba2b23190e9a39b7bfe75ec5895e885dc9ffaccd)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm