[libc] Enable linting by default in libc

[michaelrj-google]

Issues that could be caught by the linter are causing issues for real-world users, so finishing the linting work needs to be a priority.

Steps:
First we need to make sure that our lint rules are complete and include fix-it annotations so they can be applied automatically (e.g. https://reviews.llvm.org/D157164)

Second we need to apply these lint rules to clean up the existing code, the individual parts are tracked by these issues: #60984 #61076 #61078

Third we need to make sure the lint bot (https://lab.llvm.org/staging/#/builders/242) is cycling green, then move it out of staging.

Fourth we need to make sure that clang-tidy runs by default (possibly only runtimes builds?) to help avoid regressions. Additionally we need to raise awareness of this with existing libc developers.

Additionally it would be optimal to have a presubmit lint check instead of just postsubmit.

[llvmbot]

@llvm/issue-subscribers-libc

[llvmbot]

@llvm/issue-subscribers-infrastructure

[michaelrj-google]

Assigned to Nick since he's actively working on this.