[clang-tidy] Enable modernize-concat-nested-namespaces also on headers

For some reason the initial implementation of the check had an explicit check
for the main file to avoid being applied in headers. This diff removes this
check and add a test for the check on a header.

Similar approach was proposed in D61989 but review there got stuck.

Test Plan: added new test case

Differential Revision: https://reviews.llvm.org/D97563