[clang][www] Change 'clang-test' to 'check-clang' on the hacking webpage

This change is https://reviews.llvm.org/D49549 ported to the monorepo layout.

Patch by Arnaud Coomans.

Patch title and message:
```
Change 'clang-test' to 'check-clang' on the hacking webpage

The command to run tests was previously changed from 'clang-test' to 'check-clang'; with 'clang-test' remaining available as a legacy alias (see git-svn-id: https://reviews.llvm.org/rC159483 91177308-0d34-0410-b5e6-96231b3b80d8).

This commit changes the www documentation accordingly.
```

clang/www/hacking.html

@@ -126,7 +126,7 @@ <h3 id="testingNonWindows">Testing on Unix-like Systems</h3>
   about what is being run.</p>
 
   <p>If you built LLVM and Clang using CMake, the test suite can be run
-  with <tt>make clang-test</tt> from the top-level LLVM directory.</p>
+  with <tt>make check-clang</tt> from the top-level LLVM directory.</p>
 
   <p>The tests primarily consist of a test runner script running the compiler
   under test on individual test files grouped in the directories under the
@@ -193,8 +193,8 @@ <h3 id="testingWindows">Testing using Visual Studio on Windows</h3>
   to CMake explicitly.</p>
 
   <p>The cmake build tool is set up to create Visual Studio project files
-  for running the tests, "clang-test" being the root.  Therefore, to
-  run the test from Visual Studio, right-click the clang-test project
+  for running the tests, "check-clang" being the root.  Therefore, to
+  run the test from Visual Studio, right-click the check-clang project
   and select "Build".</p>
 
   <p>