[clangd] Add integration test for crash handling
This replaces the test removed in
51be7061d025139ba66869d5d99c7157a3ae9edd
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