[Test] Defined target-abi-stability string.

For FileCheck that vary on this basis.

Author: nate-chandler

test/lit.cfg

@@ -962,12 +962,14 @@ if run_vendor == 'apple':
         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.abi_stability = 'stable'
         config.available_features.add('swift_stable_abi')
     else:
         # iOS 12.2 does not support 32-bit targets, so we cannot run tests that
         # want to deploy to an iOS that has Swift in the OS.
         if run_os == 'ios' and run_ptrsize == '32':
             pre_stable_version = '10'
+            config.abi_stability = 'unstable'
         else:
             config.available_features.add('swift_stable_abi')
             PRE_STABLE_VERSION = {
@@ -1015,12 +1017,14 @@ if run_vendor == 'apple':
         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)
+        config.abi_stability = 'stable'
 
 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
+    config.abi_stability = 'unstable'
 
 # 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.
@@ -1040,6 +1044,9 @@ config.substitutions.append(('%target-next-stable-abi-triple',
 config.substitutions.append(('%target-future-triple',
                              config.future_triple))
 
+config.substitutions.append(('%target-abi-stability',
+                             config.abi_stability))
+
 config.substitutions.append(('%target-cpu', run_cpu))
 
 target_os_abi = run_os