[X86] Add __get_cpuid_count to cpuid.h. Update __get_cpuid to check the maximum level support before accessing the leaf. Rename level to leaf everywhere.
[X86] Add __get_cpuid_count to cpuid.h. Update __get_cpuid to check the maximum level support before accessing the leaf. Rename level to leaf everywhere.

This matches gcc behavior.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@307506 91177308-0d34-0410-b5e6-96231b3b80d8

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