[clangd] Add integration test for crash handling

This replaces the test removed in 51be7061
It is more principled and tests more critical cases: a crash while parsing.

We need two pieces of plumbing:
 - a way to re-enable the crashing #pragmas via a flag, to test parse crashes
 - a bit of reshuffling around ASTWorker execution so that we set up the
   crash handler in both sync/async modes.
   Sync mode is useful for debugging, so I tested both.

Differential Revision: https://reviews.llvm.org/D112565