Add new XCODE_VERSION toolchain env var (#222)

This can be used to invalidate the toolchain when your Xcode version
changes but the path to Xcode doesn't. This is required if you want to
avoid non-hermetic issues when switching Xcode versions. Previously we
used USE_CLANG_CL for this since it was otherwise ignored, but we should
move to this one which makes it more clear what the purpose is.

crosstool/setup.bzl

@@ -45,10 +45,11 @@ def _apple_cc_autoconf_impl(repository_ctx):
 _apple_cc_autoconf = repository_rule(
     environ = [
         _DISABLE_ENV_VAR,
-        "USE_CLANG_CL",  # Kept as a hack for those who rely on this invaliding the toolchain
         "DEVELOPER_DIR",  # Used for making sure we use the right Xcode for compiling toolchain binaries
         "GCOV",  # TODO: Remove this
+        "USE_CLANG_CL",  # Kept as a hack for those who rely on this invaliding the toolchain
         "USER",  # Used to allow paths for custom toolchains to be used by C* compiles
+        "XCODE_VERSION",  # Force re-computing the toolchain by including the current Xcode version info in an env var
     ],
     implementation = _apple_cc_autoconf_impl,
     configure = True,