[clangd] Reland DefineInline action availability checks

Summary:
Introduces DefineInline action and initial version of availability
checks.

Reviewers: sammccall, ilya-biryukov, hokein

Reviewed By: hokein

Subscribers: thakis, usaxena95, mgorny, ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65433