Move STDLIB_VARIANT feature in lit.cfg

Author: Azoy

test/lit.cfg

@@ -778,6 +778,8 @@ config.available_features.add("VENDOR=" + run_vendor)
 
 config.available_features.add("SWIFT_VERSION=" + swift_version)
 
+config.available_features.add("STDLIB_VARIANT={}".format(config.variant_suffix[1:]))
+
 if "optimized_stdlib" in config.available_features:
   config.available_features.add("optimized_stdlib_" + run_cpu)
 

test/lit.site.cfg.in

@@ -73,8 +73,6 @@ config.testing_against_host_compiler = \
 
 # Please remember to handle empty strings and/or unset variables correctly.
 
-config.available_features.add("STDLIB_VARIANT={}".format(config.variant_suffix[1:]))
-
 if "@SWIFT_ASAN_BUILD@" == "TRUE":
     config.available_features.add("asan")
 else: