[clangd] Report diagnostics even if WantDiags::No AST was reused

Summary:
After r338256, clangd stopped reporting diagnostics if WantDiags::No request
is followed by a WantDiags::Yes request but the AST can be reused.

Reviewers: ioeric

Reviewed By: ioeric

Subscribers: javed.absar, MaskRay, jkorous, arphaman, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D50045

llvm-svn: 338361