[lit] Support reading arguments from a file

This allows reading arguments from file using the response file syntax.
We would like to use this in the LLVM build to pass test suites from
subbuilds.

Differential Revision: https://reviews.llvm.org/D132437

(cherry picked from commit b52820edec6f3d95edef7859a56ef30878b08382)

llvm/docs/CommandGuide/lit.rst

@@ -43,6 +43,9 @@ parsing options from the command line.  ``LIT_OPTS`` is primarily useful for
 supplementing or overriding the command-line options supplied to :program:`lit`
 by ``check`` targets defined by a project's build system.
 
+:program:`lit` can also read options from response files which are specified as
+inputs using the ``@path/to/file.rsp`` syntax.
+
 Users interested in the :program:`lit` architecture or designing a
 :program:`lit` testing implementation should see :ref:`lit-infrastructure`.
 

llvm/utils/lit/lit/cl_arguments.py

@@ -16,7 +16,7 @@ class TestOrder(enum.Enum):
 
 
 def parse_args():
-    parser = argparse.ArgumentParser(prog='lit')
+    parser = argparse.ArgumentParser(prog='lit', fromfile_prefix_chars='@')
     parser.add_argument('test_paths',
             nargs='+',
             metavar="TEST_PATH",