lit.cfg: add a %target-future-triple substitution.

Expands to, e.g. `x86_64-apple-macosx99.99`
This is useful to test features which are available in a "future" runtime.

Author: eeckstein

test/lit.cfg

@@ -778,6 +778,7 @@ if run_vendor == 'apple':
         config.stable_abi_triple = '%s-%s-ios13.1-macabi' % (run_cpu, run_vendor)
         config.pre_stable_abi_triple = config.stable_abi_triple
         config.next_stable_abi_triple = config.stable_abi_triple
+        config.future_abi_triple = '%s-%s-ios99-macabi' % (run_cpu, run_vendor)
         config.available_features.add('swift_stable_abi')
     else:
         # iOS 12.2 does not support 32-bit targets, so we cannot run tests that
@@ -818,11 +819,22 @@ if run_vendor == 'apple':
         next_stable_version = NEXT_STABLE_VERSION.get(run_os, '')
         config.next_stable_abi_triple = '%s-%s-%s%s%s' % (run_cpu, run_vendor, run_os,
                                                           next_stable_version, run_environment)
+        FUTURE_VERSION = {
+            'macosx': '99.99',
+            'ios': '99.0',
+            'maccatalyst': '99.99',
+            'tvos': '99.0',
+            'watchos': '9.99.0'
+        }
+        future_version = FUTURE_VERSION.get(run_os, '')
+        config.future_triple = '%s-%s-%s%s%s' % (run_cpu, run_vendor, run_os,
+                                                 future_version, run_environment)
 
 else:
     config.pre_stable_abi_triple = config.variant_triple
     config.stable_abi_triple = config.variant_triple
     config.next_stable_abi_triple = config.variant_triple
+    config.future_triple = config.variant_triple
 
 # On Apple platforms, this substitution names the maximum OS version *without*
 # Swift in the OS. On non-Apple platforms this is equivalent to %target-triple.
@@ -839,6 +851,9 @@ config.substitutions.append(('%target-stable-abi-triple',
 config.substitutions.append(('%target-next-stable-abi-triple',
                              config.next_stable_abi_triple))
 
+config.substitutions.append(('%target-future-triple',
+                             config.future_triple))
+
 # Sanitizers are not supported on iOS7, yet all tests are configured to start
 # testing with the earliest supported platform, which happens to be iOS7 for
 # Swift.