[test/dsymutil] Suppress LeakSanitizer on ARM dsymutil tests

Just like on X86, we're getting reports of bogus leaks from
CoreFoundation in:

[_CFXPreferences(SourceAdditions) withSourceForIdentifier:user:byHost:container:cloud:perform:]

rdar://63238710

llvm/test/tools/dsymutil/ARM/lit.local.cfg

@@ -3,4 +3,8 @@ if not 'ARM' in config.root.targets:
 if not 'AArch64' in config.root.targets:
     config.unsupported = True
 
+if config.llvm_use_sanitizer:
+    suppr = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'suppressions.txt')
+    config.environment['LSAN_OPTIONS'] = 'suppressions={}'.format(suppr)
+
 config.suffixes = ['.test', '.cpp', '.c']