[test] Don't run still-crashing compiler_crashers under no-asserts builds.

Per discussion on the list. From now on, when you fix a compiler_crashers
test case, move it to compiler_crashers_fixed so that we get no-asserts and
ASan coverage.

Swift SVN r27055

validation-test/compiler_crashers/lit.local.cfg

@@ -1,2 +1,4 @@
 if "asan" in config.available_features:
     config.unsupported = True
+if "asserts" not in config.available_features:
+    config.unsupported = True