Remove `requires` for x86 CPU features.
Remove `requires` for x86 CPU features.

Ever since the target attributes change, we don't need to guard these
headers with `requires`. Actually it's a bit worse, because if we do
then they are included textually under the covers, causing declarations
to appear in submodules they aren't supposed to be in.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@240720 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang