[clangd] Respect .clang-tidy ExtraArgs (-Wfoo only) when producing diagnostics

This mechanism is used almost exclusively to enable extra warnings in clang-tidy
using ExtraArgs=-Wfoo, Checks="clang-diagnostic-foo".
Its presence is a strong signal that these flags are useful.

We choose not to actually emit them as clang-tidy diagnostics, but under their
"main" name - this ensures we show the same diagnostic in a consistent way.

We don't add the ExtraArgs to the compile command in general, but rather just
handle the -W<group> flags, which is the common case and avoids unexpected
side-effects.
And we only do this for the main file parse, when producing diagnostics.

Differential Revision: https://reviews.llvm.org/D116147