[libc++] Rename check-cxx-deps to cxx-test-depends for consistency

[ldionne]

Summary

Several subprojects have targets that do the same thing, and they all
follow the same naming convention: llvm-test-depends, clang-test-depends,
lld-test-depends, etc.

This makes libc++ consistent with other LLVM projects.
Thanks to Duncan Exon Smith for noticing and suggesting the change.

[dexonsmith]

LGTM!

Can / should this be added to the catch-all test-depends as well? (I thought llvm-test-depends and clang-test-depends were both added there somewhere, although I admit I just the code and I don't see it...) Regardless, this could be done separately / later if it makes sense to do.

[ldionne]

In D94499#2493557, @dexonsmith wrote:

LGTM!

Can / should this be added to the catch-all test-depends as well? (I thought llvm-test-depends and clang-test-depends were both added there somewhere, although I admit I just the code and I don't see it...) Regardless, this could be done separately / later if it makes sense to do.

My preference would be not to do that. The reason is that we want to have different build requirements for libc++ and Clang or LLVM. For example, we want to move towards requiring a more recent compiler. Running the libc++ build (and/or tests) by default when setting up LLVM would make that more difficult.

libcxx/docs/TestingLibcxx.rst

 .. _LIT Command Guide: https://llvm.org/docs/CommandGuide/lit.html
 
 Usage
 -----
 
 After building libc++, you can run parts of the libc++ test suite by simply
 running ``llvm-lit`` on a specified test or directory. If you're unsure
 whether the required libraries have been built, you can use the
-`check-cxx-deps` target. For example:
+`cxx-test-depends` target. For example:
 
 .. code-block:: bash
 
   $ cd <monorepo-root>
-  $ make -C <build> check-cxx-deps # If you want to make sure the targets get rebuilt
+  $ make -C <build> cxx-test-depends # If you want to make sure the targets get rebuilt
   $ <build>/bin/llvm-lit -sv libcxx/test/std/re # Run all of the std::regex tests
   $ <build>/bin/llvm-lit -sv libcxx/test/std/depr/depr.c.headers/stdlib_h.pass.cpp # Run a single test
   $ <build>/bin/llvm-lit -sv libcxx/test/std/atomics libcxx/test/std/threads # Test std::thread and std::atomic
 
 Sometimes you'll want to change the way LIT is running the tests. Custom options
 can be specified using the `--param=<name>=<val>` flag. The most common option
 you'll want to change is the standard dialect (ie -std=c++XX). By default the
 test suite will select the newest C++ dialect supported by the compiler and use

 configuration, simply point the CMake build to it using
 ``-DLIBCXX_TEST_CONFIG=<path-to-site-config>``, and that site configuration
 will be used instead. That file can use CMake variables inside it to make
 configuration easier.
 
    .. code-block:: bash
 
      $ cmake <options> -DLIBCXX_TEST_CONFIG=<path-to-site-config>
-     $ make -C <build> check-cxx-deps
+     $ make -C <build> cxx-test-depends
      $ <build>/bin/llvm-lit -sv libcxx/test # will use your custom config file
 
 
 LIT Options
 ===========
 
 :program:`lit` [*options*...] [*filenames*...]
 

libcxx/test/CMakeLists.txt

 if (LIBCXX_INCLUDE_TESTS)
   include(AddLLVM) # for configure_lit_site_cfg and add_lit_target
 
   configure_lit_site_cfg(
     "${LIBCXX_TEST_CONFIG}"
     ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
     MAIN_CONFIG "${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py")
 
-  add_custom_target(check-cxx-deps
+  add_custom_target(cxx-test-depends
     DEPENDS cxx ${LIBCXX_TEST_DEPS}
     COMMENT "Builds dependencies required to run the test suite.")
 
   add_lit_target(check-cxx
     "Running libcxx tests"
     ${CMAKE_CURRENT_BINARY_DIR}
-    DEPENDS check-cxx-deps
+    DEPENDS cxx-test-depends
     PARAMS "${LIBCXX_TEST_PARAMS}")
 endif()
 
 if (LIBCXX_GENERATE_COVERAGE)
   include(CodeCoverage)
   set(output_dir "${CMAKE_CURRENT_BINARY_DIR}/coverage")
   set(capture_dirs
       "${LIBCXX_LIB_CMAKEFILES_DIR}/cxx_objects.dir/"