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

[LegalizeAdulthood]

Summary

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.

[aaron.ballman]

This sounds like a sensible direction to me. I spot-checked the changes and they all seem reasonable. Giving my LG, but please wait a bit before landing in case someone else has the chance to spot check as well.

[LegalizeAdulthood]

In D128072#3598634, @aaron.ballman wrote:

This sounds like a sensible direction to me. I spot-checked the changes and they all seem reasonable. Giving my LG, but please wait a bit before landing in case someone else has the chance to spot check as well.

Testing was:

• check-clang-extra passed
• add_new_check.py added the boiler plate test code in the correct place

[njames93]

Just noticed something nice about this, cmake targets are generated for each module
Should speed up development as you now only need to run the lit tests for the module that you are working on check-clang-extra-clang-tidy-checkers-misc etc