clang: improve lit testing docs (#178244)

The LLVM Integrated Tester now generates an "easy to use" script to run
clang tests. It is no longer needed to pass all of the commandline
arguments to it. This PR simplifies the documentation a little by
removing the unneeded commandline arguments and adding a link to the lit
man page.

clang/www/hacking.html

@@ -207,55 +207,48 @@
   <!--=====================================================================-->
 
   <p>If you want more control over how the tests are run, it may
-  be convenient to run the test harness on the command-line directly. Before
-  running tests from the command line, you will need to ensure that
-  <tt>lit.site.cfg</tt> files have been created for your build.  You can do
-  this by running the tests as described in the previous sections. Once the
-  tests have started running, you can stop them with control+C, as the
-  files are generated before running any tests.</p>
+  be convenient to run the test harness on the command-line directly. Running
+  the <tt>check-clang</tt> build target will generate a script to start the
+  <a href="https://llvm.org/docs/CommandGuide/lit.html">LLVM Integrated
+  Tester</a> (<tt>lit</tt>) that can be used to run tests for your
+  current configuration. Once the tests have started running, you can stop
+  them with <tt>control+C</tt>, as the files are generated before running any
+  tests.</p>
 
-  <p>Once that is done, to run all the tests from the command line,
-  execute a command like the following:</p>
+  <p>Once that is done, all the tests can be executed from the command line
+  by running the generated <tt>llvm-lit</tt> script as follows:</p>
 
   <pre>
-  python (path to llvm)\llvm\utils\lit\lit.py -sv
-  --param=build_mode=Win32 --param=build_config=Debug
-  --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
- (path to llvm)\llvm\tools\clang\test
-</pre>
-
-  <p>For CMake builds e.g. on Windows with Visual Studio, you will need
-  to specify your build configuration (Debug, Release, etc.) via
-  <tt>--param=build_config=(build config)</tt>.  You may also need to specify
-  the build mode (Win32, etc) via <tt>--param=build_mode=(build mode)</tt>.</p>
-
-  <p>Additionally, you will need to specify the lit site configuration which
-  lives in (build dir)\tools\clang\test, via
-  <tt>--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg</tt>.
-  </p>
-
-  <p>To run a single test:</p>
+  (build dir)\bin\llvm-lit (path to llvm)\clang\test
+  </pre>
 
+  For example; if you have a Ninja build in the
+  <tt>llvm-project\build_ninja</tt> directory, the command to execute from the
+  <tt>llvm-project</tt> directory would be:
   <pre>
-  python (path to llvm)\llvm\utils\lit\lit.py -sv
-  --param=build_mode=Win32 --param=build_config=Debug
-  --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
-  (path to llvm)\llvm\tools\clang\test\(dir)\(test)
-</pre>
+  build_ninja\bin\llvm-lit clang\test 
+  </pre>
 
-  <p>For example:</p>
 
+  Or, for a Visual Studio Debug build in the <tt>llvm-project\build</tt>
+  directory, the lit start command to execute from the <tt>llvm-project</tt>
+  directory would be:
   <pre>
-  python C:\Tools\llvm\utils\lit\lit.py -sv
-  --param=build_mode=Win32 --param=build_config=Debug
-  --param=clang_site_config=C:\Tools\build\tools\clang\test\lit.site.cfg
-  C:\Tools\llvm\tools\clang\test\Sema\wchar.c
-</pre>
+  build\Debug\bin\llvm-lit clang\test
+  </pre>
 
-  <p>The -sv option above tells the runner to show the test output if
-  any tests failed, to help you determine the cause of failure.</p>
+  <p>You can run a single test or all tests in a specific folder by providing
+  the target test or folder to <tt>lit</tt>. For example, we can run the
+  <tt>wchar.c</tt> test:</p>
+  <pre>
+    build_ninja\bin\llvm-lit clang\test\Sema\wchar.c
+  </pre>
+  <p>or all tests in the <tt>Sema</tt> folder:</p>
+  <pre>
+    build_ninja\bin\llvm-lit clang\test\Sema
+  </pre>
 
-  <p>You can also pass in the --no-progress-bar option if you wish to disable
+  <p>Pass in the <tt>--no-progress-bar</tt> option if you wish to disable
   progress indications while the tests are running.</p>
 
   <p>Your output might look something like this:</p>