[clang-tidy] Organize test files into subdirectories by module (NFC)

Eliminate clutter by reorganizing the Lit test files for clang-tidy:
- Move checkers/<module>-* to checkers/<module>/*.
- Move module specific inputs from Inputs to <module>/Inputs.  Remove
  any module prefix from the file or subdirectory name as they are no
  longer needed.
- Introduce a Lit substitution %clang_tidy_headers for the system
  headers in checkers/Inputs/Headers and use this throughout.  This
  avoids referencing system headers through a relative path to the
  parent directory and makes it clear that these fake system headers are
  shared among all modules.
- Update add_new_check.py to follow the above conventions when creating
  the boiler plate test files for a new check.
- Update Contributing.rst to describe per-module Inputs directory and
  fix link to test source code.

Differential Revision: https://reviews.llvm.org/D128072

clang-tools-extra/clang-tidy/add_new_check.py

@@ -278,8 +278,9 @@ def add_release_notes(module_path, module, check_name):
 # Adds a test for the check.
 def write_test(module_path, module, check_name, test_extension):
   check_name_dashes = module + '-' + check_name
-  filename = os.path.normpath(os.path.join(module_path, '../../test/clang-tidy/checkers',
-                                           check_name_dashes + '.' + test_extension))
+  filename = os.path.normpath(os.path.join(
+    module_path, '..', '..', 'test', 'clang-tidy', 'checkers',
+    module, check_name + '.' + test_extension))
   print('Creating %s...' % filename)
   with io.open(filename, 'w', encoding='utf8', newline='\n') as f:
     f.write("""// RUN: %%check_clang_tidy %%s %(check_name_dashes)s %%t

clang-tools-extra/docs/clang-tidy/Contributing.rst

@@ -582,7 +582,7 @@ to incorrect parts of the input. Use ``[[@LINE+X]]``/``[[@LINE-X]]``
 substitutions and distinct function and variable names in the test code.
 
 Here's an example of a test using the ``check_clang_tidy.py`` script (the full
-source code is at `test/clang-tidy/google-readability-casting.cpp`_):
+source code is at `test/clang-tidy/checkers/google/readability-casting.cpp`_):
 
 .. code-block:: c++
 
@@ -632,12 +632,18 @@ most frequent pitfalls are macros and templates:
    expansions/instantiations.
 
 If you need multiple files to exercise all the aspects of your check, it is
-recommended you place them in a subdirectory named for the check under ``Inputs``.
-This keeps the test directory from getting cluttered.
+recommended you place them in a subdirectory named for the check under the ``Inputs``
+directory for the module containing your check.  This keeps the test directory from
+getting cluttered.
+
+If you need to validate how your check interacts with system header files, a set
+of simulated system header files is located in the ``checkers/Inputs/Headers``
+directory.  The path to this directory is available in a lit test with the variable
+``%clang_tidy_headers``.
 
 .. _lit: https://llvm.org/docs/CommandGuide/lit.html
 .. _FileCheck: https://llvm.org/docs/CommandGuide/FileCheck.html
-.. _test/clang-tidy/google-readability-casting.cpp: https://reviews.llvm.org/diffusion/L/browse/clang-tools-extra/trunk/test/clang-tidy/google-readability-casting.cpp
+.. _test/clang-tidy/checkers/google/readability-casting.cpp: https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/test/clang-tidy/checkers/google/readability-casting.cpp
 
 Out-of-tree check plugins
 -------------------------

clang-tools-extra/test/clang-tidy/checkers/altera/kernel-name-restriction.cpp

@@ -1,5 +1,5 @@
-// RUN: %check_clang_tidy %s altera-kernel-name-restriction %t -- -- -I%S/Inputs/altera-kernel-name-restriction
-// RUN: %check_clang_tidy -check-suffix=UPPERCASE %s altera-kernel-name-restriction %t -- -- -I%S/Inputs/altera-kernel-name-restriction/uppercase -DUPPERCASE
+// RUN: %check_clang_tidy %s altera-kernel-name-restriction %t -- -- -I%S/Inputs/kernel-name-restriction
+// RUN: %check_clang_tidy -check-suffix=UPPERCASE %s altera-kernel-name-restriction %t -- -- -I%S/Inputs/kernel-name-restriction/uppercase -DUPPERCASE
 
 #ifdef UPPERCASE
 // The warning should be triggered regardless of capitalization

clang-tools-extra/test/clang-tidy/checkers/bugprone/argument-comment.cpp

@@ -1,4 +1,4 @@
-// RUN: %check_clang_tidy %s bugprone-argument-comment %t -- -- -I %S/Inputs/bugprone-argument-comment
+// RUN: %check_clang_tidy %s bugprone-argument-comment %t -- -- -I %S/Inputs/argument-comment
 
 // FIXME: clang-tidy should provide a -verify mode to make writing these checks
 // easier and more accurate.