[clang-tidy] follow-up on misc-definitions-in-header check.

Summary:
A follow-up on D34449:
* add `-std=c++11` to `.hpp` file by default.
* add constexpr function to test and doc.

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: JDevlieghere, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D34771

llvm-svn: 306650