lit: warn when passed invalid pathname

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'

Differential Revision: http://reviews.llvm.org/D4097

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210597 91177308-0d34-0410-b5e6-96231b3b80d8

utils/lit/lit/discovery.py

@@ -200,9 +200,7 @@ def find_tests_for_inputs(lit_config, inputs):
     # 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:
@@ -211,6 +209,10 @@ def find_tests_for_inputs(lit_config, inputs):
                         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 = []