[clang-tidy] modified identifier naming case to use CT_AnyCase for ignoring case style

Summary:
Using CaseType::CT_AnyCase for selected identifier results in inheriting case style setting from more basic identifier type.

This patch changes CT_AnyCase behavior to ignore case style of specified identifier. If case style was not set, llvm::Optional will be used for keeping this information (llvm::Optional<>::hasVal), thus CT_AnyCase will no longer mean more general identifier style should be used.

This eliminates false-positives when naming convention is not clear for specific areas of code (legacy, third party) or for selected types.

Reviewers: berenm, alexfh

Reviewed By: alexfh

Subscribers: cfe-commits, JDevlieghere

Differential Revision: https://reviews.llvm.org/D30931

llvm-svn: 298499