[lit] Flush stderr manually on Windows after printing messages

When run in a git bash terminal, sys.stderr isn't flushed implicitly
after printing each line. Manually flush it after each printout,
to avoid getting broken/misordered output.

A similar fix had been done in the old libcxx test config, committed
as part of 7e3ee09 / D28725; this
generalizes the fix, making it available in the new libcxx test
configs too, and for any other test that uses lit_config.note().

Differential Revision: https://reviews.llvm.org/D115761

libcxx/utils/libcxx/test/config.py

@@ -159,7 +159,6 @@ def print_config_info(self):
         self.lit_config.note("Running against the C++ Library at {}".format(self.cxx_runtime_root))
         self.lit_config.note("Linking against the ABI Library at {}".format(self.abi_library_root))
         self.lit_config.note("Running against the ABI Library at {}".format(self.abi_runtime_root))
-        sys.stderr.flush()  # Force flushing to avoid broken output on Windows
 
     def get_test_format(self):
         from libcxx.test.format import LibcxxTestFormat

llvm/utils/lit/lit/LitConfig.py

@@ -169,6 +169,11 @@ def _write_message(self, kind, message):
         line = inspect.getlineno(f)
         sys.stderr.write('%s: %s:%d: %s: %s\n' % (self.progname, file, line,
                                                   kind, message))
+        if self.isWindows:
+            # In a git bash terminal, the writes to sys.stderr aren't visible
+            # on screen immediately. Flush them here to avoid broken/misoredered
+            # output.
+            sys.stderr.flush()
 
     def note(self, message):
         if not self.quiet: