[asan] Add lit feature to indicate compiler_rt's shadow scale value

[waltl]

Summary

This will be used to mark tests that require a specific shadow scale.

[waltl]

Change approach to only override shadow scale in compiler_rt

[vitalybuka]

compiler-rt/test/lit.common.cfg

332

can we see a patch where it's used?

[waltl]

compiler-rt/test/lit.common.cfg

332

https://reviews.llvm.org/D39774

[vitalybuka]

compiler-rt/test/lit.common.cfg

332

WDYT about "config.available_features.add("shadow-scale-%s" % (config.asan_shadow_scale if not empty else 3))"?

So you can replace " UNSUPPORTED: custom-shadow-scale" with " REQUIRES: shadow-scale-3"
This allows to add some scale-5 specific tests

[waltl]

compiler-rt/test/lit.common.cfg

332

That sounds much better. Thanks.

[waltl]

Address CR comments

compiler-rt/test/lit.common.cfg

if config.host_os == 'Darwin':
   config.default_sanitizer_opts += ['log_to_syslog=0']
 elif config.android:
   config.default_sanitizer_opts += ['abort_on_error=0']
 
 # Allow tests to use REQUIRES=stable-runtime.  For use when you cannot use XFAIL
 # because the test hangs or fails on one configuration and not the other.
 if config.android or (config.target_arch not in ['arm', 'armhf', 'aarch64']):
   config.available_features.add('stable-runtime')
+
+# Detect custom shadow scale.
+if config.asan_shadow_scale != '' and config.asan_shadow_scale != '3':
+  config.available_features.add("custom-shadow-scale")