inline stmt attribute diagnosing in templates

inline stmt attribute diagnosing in templates

D146089's author discovered that our diagnostics for always/no inline
would null-dereference when used in a template. He fixed that by
skipping in the dependent case.

This patch makes sure we diagnose these after a template instantiation.
It also adds infrastructure for other statement attributes to add
checking/transformation.

Differential Revision: https://reviews.llvm.org/D146323