Add swift_build/swift-build to swift_build_support/toolchain.py

This exposes `swift-build` from toolchains, which allows products to use this tool directly when building

Author: MaxDesiatov

utils/swift_build_support/swift_build_support/toolchain.py

@@ -70,6 +70,7 @@ def _getter(self):
     _register("ar", "ar")
 _register("sccache", "sccache")
 _register("swiftc", "swiftc")
+_register("swift_build", "swift-build")
 
 
 class Darwin(Toolchain):