[scudo][standalone] Add flags & related parsers

Summary:
As with other Sanitizers, and the current version of Scudo, we can
provide flags in differents way: at compile time, through a weak
function, through an environment variable.

This change adds support for the configuration flags, and the string
parsers. Those are fairly similar to the sanitizer_common way of doing
things.

Reviewers: morehouse, hctim, vitalybuka

Reviewed By: morehouse, vitalybuka

Subscribers: mgorny, delcypher, jdoerfert, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D59597

llvm-svn: 358011
GitOrigin-RevId: 7e2b15382c270ff868fb48c93dd1f125c543d289
diff --git a/flags.h b/flags.h
new file mode 100644
index 0000000..edd39a1
--- /dev/null
+++ b/flags.h
@@ -0,0 +1,30 @@
+//===-- flags.h -------------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SCUDO_FLAGS_H_
+#define SCUDO_FLAGS_H_
+
+#include "internal_defs.h"
+
+namespace scudo {
+
+struct Flags {
+#define SCUDO_FLAG(Type, Name, DefaultValue, Description) Type Name;
+#include "flags.inc"
+#undef SCUDO_FLAG
+  void setDefaults();
+};
+
+Flags *getFlags();
+void initFlags();
+class FlagParser;
+void registerFlags(FlagParser *Parser, Flags *F);
+
+} // namespace scudo
+
+#endif // SCUDO_FLAGS_H_