Guard lit_config.quiet for lit versions that removed it.

test/lit.cfg reads lit_config.quiet directly, but upstream lit has
dropped that attribute from LitConfig: --quiet survives only as a
command line flag gating the result summary, and LitConfig.note() is now
unconditional. Where the attribute is gone, parsing the config raises
AttributeError, so the suite fails to load and no test is discovered at
all.

CI does not hit this yet, because it drives the suite with the lit
package from PyPI -- currently 18.1.8, which still has the attribute --
and that version is independent of the clang-runtime a given row builds
against. It does bite when running the suite with the in-tree llvm-lit
of a recent LLVM, and it will bite CI once "pip install lit" resolves to
a version that dropped it. Read the flag defensively so it works either
way.

Author: vgvassilev

test/lit.cfg

@@ -246,7 +246,7 @@ def inferClang(PATH):
 config.clang = inferClang(config.environment['PATH']).replace('\\', '/')
 config.cladlib = inferCladLib(config.environment['PATH']).replace('\\', '/')
 
-if not lit_config.quiet:
+if not getattr(lit_config, 'quiet', False):
     lit_config.note('using clang: %r' % config.clang)
     lit_config.note('using auto diff lib: %r' % config.cladlib)