[build-script] Introduce infrastructure for auto-prefixing stage2 options.

This will let me introduce a stage generic swift implementation that adds a
postfix '_stage2' to certain stage specific options. This ensures we can have a
single swift build-script product implementation for both stage1 and stage2
compilers.

Author: gottesmm

utils/swift_build_support/swift_build_support/compiler_stage.py

@@ -0,0 +1,32 @@
+# ===--- compiler_stage.py -----------------------------------------------===#
+#
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https:#swift.org/LICENSE.txt for license information
+# See https:#swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+#
+# ===---------------------------------------------------------------------===#
+
+class StageArgs(object):
+    def __init__(self, stage, args):
+        self.stage = stage
+        self.args = args
+
+    def __getattr__(self, key):
+        real_key = '{}{}'.format(key, self.stage.postfix)
+        if not hasattr(self.args, real_key):
+            return None
+        return getattr(self.args, real_key)
+
+
+class Stage(object):
+    def __init__(self, identifier, postfix=""):
+        self.identifier = identifier
+        self.postfix = postfix
+
+
+STAGE_1 = Stage(1, "")
+STAGE_2 = Stage(2, "_stage2")