[clangd] Support external throttler for preamble builds

[clangd] Support external throttler for preamble builds

Building preambles is the most resource-intensive thing clangd does, driving
peak RAM and sustained CPU usage.

In a hosted environment where multiple clangd instances are packed into the same
container, it's useful to be able to limit the *aggregate* resource peaks.

Differential Revision: https://reviews.llvm.org/D129100