[clangd] Update TUStatus api to accommodate preamble thread

Summary:
TUStatus api had a single thread in mind. This introudces a section
action to represent state of the preamble thread. In the file status extension,
we keep old behavior almost the same. We only prepend current task with a
`parsing includes` if preamble thread is working. We omit the idle thread in the
output unless both threads are idle.

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76304