[docs][FileCheck] add link to VerifyDiagnosticConsumer docs

[nickdesaulniers]

Summary

I find the VerifyDiagnosticConsumer docs hard to find otherwise, and I
think most folks writing tests for Clang end up on the FileCheck page,
regardless of whether the test they're trying to write uses FileCheck or
just -verify.

[aaron.ballman]

It seems a bit weird to me for the FileCheck docs to be talking about -verify at all. FileCheck is a standalone program in LLVM (so the reader may not be considering Clang at all), and -verify is a clang command line option. I did a bit of googling and it doesn't look like we have a particularly good testing page for Clang like we do for LLVM (https://llvm.org/docs/TestingGuide.html) -- the closest we have is https://clang.llvm.org/hacking.html#testing. I sort of wonder if this is a sign that we should add one?

[nickdesaulniers]

Hmm, so I've created a new document, clang/docs/Testing.rst but:

$ cd llvm/build
$ cmake <usual stuff> -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_HTML=true
$ ninja docs-clang-html
[1/2] Generating html Sphinx documentation for .../llvm-project/llvm/build/tools/clang/docs/html"
FAILED: tools/clang/docs/CMakeFiles/docs-clang-html 
cd /android0/llvm-project/llvm/build/tools/clang/docs && /usr/bin/sphinx-build -b html -d /android0/llvm-project/llvm/build/tools/clang/docs/_doctrees-clang-html -q -W /android0/llvm-project/clang/docs /android0/llvm-project/llvm/build/tools/clang/docs/html

Warning, treated as error:
/android0/llvm-project/clang/docs/ClangCommandLineReference.rst:22:Duplicate explicit target name: "cmdoption-clang--prefix".
ninja: build stopped: subcommand failed.

[aaron.ballman]

In D75759#1910427, @nickdesaulniers wrote:

Hmm, so I've created a new document, clang/docs/Testing.rst but:

$ cd llvm/build
$ cmake <usual stuff> -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_HTML=true
$ ninja docs-clang-html
[1/2] Generating html Sphinx documentation for .../llvm-project/llvm/build/tools/clang/docs/html"
FAILED: tools/clang/docs/CMakeFiles/docs-clang-html 
cd /android0/llvm-project/llvm/build/tools/clang/docs && /usr/bin/sphinx-build -b html -d /android0/llvm-project/llvm/build/tools/clang/docs/_doctrees-clang-html -q -W /android0/llvm-project/clang/docs /android0/llvm-project/llvm/build/tools/clang/docs/html

Warning, treated as error:
/android0/llvm-project/clang/docs/ClangCommandLineReference.rst:22:Duplicate explicit target name: "cmdoption-clang--prefix".
ninja: build stopped: subcommand failed.

Yeah, we continually run into this "duplicate explicit target name" in the command line reference. You should pass -DSPHINX_WARNINGS_AS_ERRORS=OFF when configuring cmake, unfortunately.

llvm/docs/CommandGuide/FileCheck.rst

 
 .. code-block:: text
 
        DW_AT_location [DW_FORM_sec_offset]   (0x00000233)
        DW_AT_name [DW_FORM_strp]  ( .debug_str[0x000000c9] = "intd")
 
 letting us set the :program:`FileCheck` variable ``DLOC`` to the desired value 
 ``0x00000233``, extracted from the line immediately preceding "``intd``".
+
+Clang `-verify` Tests
+---------------------
+For clang tests using `-verify` and `expected-error`, `expected-warning`, and
+`expected-note`, see the `VerifyDiagnosticConsumer
+<https://clang.llvm.org/doxygen/classclang_1_1VerifyDiagnosticConsumer.html#details>`_
+documentation.