.clang-tidy: remove llvm-libc specific checks (this isn't llvm-libc code…)

[marcusmueller]

This disables a lot of declaration must be declared within the '__llvm_libc' namespace diagnostics. Which is desirable, since this code really isn't LLVM's libc.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[marcusmueller]

@CLAassistant That will not be necessary, this is not a code contribution.

[misery]

Thanks!