Specify simulator in target triple

I notice that simulator builds don't have the `-simulator` [environment](http://llvm.org/doxygen/classllvm_1_1Triple.html#a1778f5c464f88710033f7e11e84a9324) in the target triple. With this change, `-simulator` is added for platforms where `is_device` is false. This matches Xcode.

Closes #154.

RELNOTES: `-simulator` is now added to the target triple passed to `swiftc` (e.g., "-target x86_64-apple-ios10.3-simulator") by the Xcode toolchain when targeting simulator platforms.
PiperOrigin-RevId: 237447785

swift/internal/xcode_swift_toolchain.bzl

@@ -375,8 +375,13 @@ def _swift_apple_target_triple(cpu, platform, version):
     if platform_string == "macos":
         platform_string = "macosx"
 
-    return "{cpu}-apple-{platform}{version}".format(
+    environment = ""
+    if not platform.is_device:
+        environment = "-simulator"
+
+    return "{cpu}-apple-{platform}{version}{environment}".format(
         cpu = cpu,
+        environment = environment,
         platform = platform_string,
         version = version,
     )