[build system] If there's an OVERRIDE, don't guess the current SCM.

This makes automatic checkout work even in situations where the
current repository can't be determined, such as in the case of a 
Git tag.

llvm-svn: 306460

lldb/scripts/Xcode/build-llvm.py

@@ -43,6 +43,9 @@ def process_repo(r):
     }
 
 def XCODE_REPOSITORIES():
+    override = repo.get_override()
+    if override:
+        return [process_repo(r) for r in override]
     identifier = repo.identifier()
     if identifier == None:
         identifier = "<invalid>" # repo.find will just use the fallback file

lldb/scripts/Xcode/repo.py

@@ -21,6 +21,17 @@ def identifier():
 		pass
 	return None
 
+def get_override():
+	dir = os.path.dirname(os.path.realpath(__file__))
+	repos_dir = os.path.join(dir, "repos")
+	json_regex = re.compile(r"^.*.json$")
+	override_path = os.path.join(repos_dir, "OVERRIDE")
+	if os.path.isfile(override_path):
+		override_set = json.load(open(override_path))
+		return override_set["repos"]
+        else:
+		return None
+
 def find(identifier):
 	dir = os.path.dirname(os.path.realpath(__file__))
 	repos_dir = os.path.join(dir, "repos")