Fix the clang-on-clang build:  APFloat reports underflow whenever we get a denormal, but we only want to diagnose if we underflowed to zero.  This allows people to write constants in the denormal range.
Fix the clang-on-clang build:  APFloat reports underflow whenever we get a
denormal, but we only want to diagnose if we underflowed to zero.  This
allows people to write constants in the denormal range.



git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@92129 91177308-0d34-0410-b5e6-96231b3b80d8

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