lit: warn when passed invalid pathname

[hans]

Summary

It would previously say things like

warning: input 'test/Frontend/foo.c' contained no tests

and have the user pull their hair trying to figure out what's wrong with that
file. This patch changes the message to the much clearer:

warning: no such file or directory: 'test/Frontend/foo.c'

[• ddunbar]

LGTM

[hans]

Closed by commit rL210597 (authored by @hans).

utils/lit/lit/discovery.py

def find_tests_for_inputs(lit_config, inputs):
 
     Given a configuration object and a list of input specifiers, find all the
     tests to execute.
     """
 
     # Expand '@...' form in inputs.
     actual_inputs = []
     for input in inputs:
-        if os.path.exists(input) or not input.startswith('@'):
-            actual_inputs.append(input)
-        else:
+        if input.startswith('@'):
             f = open(input[1:])
             try:
                 for ln in f:
                     ln = ln.strip()
                     if ln:
                         actual_inputs.append(ln)
             finally:
                 f.close()
+        elif os.path.exists(input):
+            actual_inputs.append(input)
+        else:
+            lit_config.warning('no such file or directory: %r' % input)
                     
     # Load the tests from the inputs.
     tests = []
     test_suite_cache = {}
     local_config_cache = {}
     for input in actual_inputs:
         prev = len(tests)
         tests.extend(getTests(input, lit_config,