[clangd] [C++20] [Modules] Introduce GC for clangd built modules (#193973)

This patch introduces simple GC for clangd built module files to avoid
the clangd built module cache to increase infinitely.

The strategy is, in a clangd built module file cache, if the clangd
built module (we think all PCM files in clangd cache are built by
clangd) was not accessed in a time (by default 3 day, controlled by
--modules-builder-versioned-gc-threshold-seconds)，clangd will remove it.

The strategy is not perfect. e.g., I heard in some systems, the atime
was forbid or not update. But given a trade off between usability and
maintainability. I feel the current stategy is fine.

AI assisted.

GitOrigin-RevId: c49b1773b2236a759dfedd3b5b67eb11856a9cbb