[CAS] swift dependency scanning using CAS for compiler caching (#66366)

Teach swift dependency scanner to use CAS to capture the full dependencies for a build and construct build commands with immutable inputs from CAS.

This allows swift compilation caching using CAS.

test/CAS/Inputs/SwiftDepsExtractor.py

@@ -0,0 +1,34 @@
+#!/usr/bin/env python3
+#
+# Usage: SwiftDepsExtractor.py file.json ModuleName Key
+
+import json
+import sys
+
+input_json = sys.argv[1]
+module_name = sys.argv[2]
+key = sys.argv[3]
+
+mode = 'swift'
+
+if module_name.startswith('clang:'):
+    mode = 'clang'
+    module_name = module_name[6:]
+elif module_name.startswith('swiftPrebuiltExternal:'):
+    mode = 'swiftPrebuiltExternal'
+    module_name = module_name[22:]
+
+with open(input_json, 'r') as file:
+    deps = json.load(file)
+    module_names = deps['modules'][::2]
+    module_details = deps['modules'][1::2]
+    for name, detail in zip(module_names, module_details):
+        if name.get(mode, '') != module_name:
+            continue
+
+        if key in detail.keys():
+            print(detail[key])
+            break
+
+        print(detail['details'][mode][key])
+        break