Disable tidy checks with too many hits

Disable tidy checks with too many hits

Summary:
Some tidy checks have too many hits in the codebase, making it hard to spot
other results from clang-tidy, therefore rendering the tool less useful.

Two checks were disabled:
  - misc-non-private-member-variable-in-classes in the whole LLVM monorepo,
    it is very common to have those in LLVM and the style guide does not forbid
    them.
  - readability-identifier-naming in the clang subtree. There are thousands of
    violations in 'Sema.h' alone.

Before the change, 'Sema.h' had >1000 tidy warnings, after the change the number
dropped to 3 warnings (unterminated namespace comments).

Reviewers: alexfh, hokein

Reviewed By: hokein

Subscribers: llvm-commits, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D57573

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352862 91177308-0d34-0410-b5e6-96231b3b80d8