[lit] Print non-existent paths when no tests are discovered.

[varungandhi-apple]

Summary

A likely failure mode is that the developer made a typo. Showing that the path
does not exist clarifies the issue; otherwise the developer might be confused as
to why the tests are not visible to lit.py even though they can see the test
directory locally.

llvm/utils/lit/lit/main.py

lit_config = lit.LitConfig.LitConfig(
         isWindows=is_windows,
         params=params,
         config_prefix=opts.configPrefix,
         echo_all_commands=opts.echoAllCommands)
 
     discovered_tests = lit.discovery.find_tests_for_inputs(lit_config, opts.test_paths)
     if not discovered_tests:
         sys.stderr.write('error: did not discover any tests for provided path(s)\n')
+        for p in opts.test_paths:
+            if not os.path.exists(p):
+                sys.stderr.write('error: path {} does not exist\n'.format(p))
         sys.exit(2)
 
     if opts.show_suites or opts.show_tests:
         print_discovered(discovered_tests, opts.show_suites, opts.show_tests)
         sys.exit(0)
 
     if opts.show_used_features:
         features = set(itertools.chain.from_iterable(t.getUsedFeatures() for t in discovered_tests))

llvm/utils/lit/tests/selecting.py

 # RUN: %{lit} %{inputs}/discovery | FileCheck --check-prefix=CHECK-BASIC %s
 # CHECK-BASIC: Testing: 5 tests
 
 
 # Check that we exit with an error if we do not discover any tests, even with --allow-empty-runs.
 #
 # RUN: not %{lit} %{inputs}/nonexistent                    2>&1 | FileCheck --check-prefix=CHECK-BAD-PATH %s
 # RUN: not %{lit} %{inputs}/nonexistent --allow-empty-runs 2>&1 | FileCheck --check-prefix=CHECK-BAD-PATH %s
 # CHECK-BAD-PATH: error: did not discover any tests for provided path(s)
+# CHECK-BAD_PATH: error: path %{inputs}/nonexistent does not exist
 
 # Check that we exit with an error if we filter out all tests, but allow it with --allow-empty-runs.
 #
 # RUN: not %{lit} --filter 'nonexistent'                    %{inputs}/discovery 2>&1 | FileCheck --check-prefixes=CHECK-BAD-FILTER,CHECK-BAD-FILTER-ERROR %s
 # RUN:     %{lit} --filter 'nonexistent' --allow-empty-runs %{inputs}/discovery 2>&1 | FileCheck --check-prefixes=CHECK-BAD-FILTER,CHECK-BAD-FILTER-ALLOW %s
 # CHECK-BAD-FILTER: error: filter did not match any tests (of 5 discovered).
 # CHECK-BAD-FILTER-ERROR: Use '--allow-empty-runs' to suppress this error.
 # CHECK-BAD-FILTER-ALLOW: Suppressing error because '--allow-empty-runs' was specified.