[libc] Make clang-tidy use host compiler's resource dir.

Summary: When building llvm-libc with linting enabled, clang-tidy would use the resource dir of the monorepo rather then the host compiler's resource dir. This presented issues when including headers from the host compiler e.g. for sanitizers. Therefore this patch explicitly tells clang-tidy to use the host compiler's resource dir.

Reviewers: sivachandra

Reviewed By: sivachandra

Subscribers: mgorny, tschuett, ecnelises, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D80265