message: [lit] On Windows, don't error if MSVC is not in PATH.
  We had some logic backwards, and as a result if MSVC was not found
  in PATH we would throw a string concatenation exception.
diff: @@ -42,12 +42,14 @@ def _use_msvc_substitutions(config):
  # If running from a Visual Studio Command prompt (e.g. vcvars), this will
  # detect the include and lib paths, and find cl.exe and link.exe and create
  # substitutions for each of them that explicitly specify /I and /L paths
  - cl = '"' + lit.util.which('cl') + '"'
  - link = '"' + lit.util.which('link') + '"'
  + cl = lit.util.which('cl')
  + link = lit.util.which('link')
  if not cl or not link:
  return
  + cl = '"' + cl + '"'
  + link = '"' + link + '"'
  includes = os.getenv('INCLUDE', '').split(';')
  libs = os.getenv('LIB', '').split(';')
  