Change bool_constant to integral_constant.
Change bool_constant to integral_constant.

bool_constant is C++17.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@337576 91177308-0d34-0410-b5e6-96231b3b80d8

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