[clangd] Refine recoveryAST flags in clangd

so that we could start experiment for C.

Previously, these flags in clangd were only meaningful for C++. We need
to flip them for C, this patch repurpose these flags.

- if true, just set it.
- if false, just respect the value in clang.

this would allow us to keep flags on for C++, and optionally flip them on for C.

Differential Revision: https://reviews.llvm.org/D89233