CodingStandards: restrict CamelCase variable names guideline to...

CodingStandards: restrict CamelCase variable names guideline to llvm/clang/clang-tools-extra/polly/bolt

See https://discourse.llvm.org/t/top-level-clang-tidy-options-and-variablename-suggestion-on-codingstandards/58783 ,
the CamelCase variable names guideline does not reflect the truth:
flang, libc, libclc, libcxx, libcxxabi, libunwind, lld, mlir, openmp,
and pstl use camelCase. lldb uses snake_case.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D140585