[clangd] No crash on "-verify" mode.

If there is a "-verify" flag in the compile command, clangd will crash
(hit the assertion) inside the `~VerifyDiagnosticConsumer` (Looks like our
compiler invocation doesn't setup correctly?).

This patch disables the verify mode as it is rarely useful in clangd.

Differential Revision: https://reviews.llvm.org/D91777