Avoid unnecessary allocations due to tracing utilities

While ticky-profiling the typechecker I noticed that hundreds of
millions of SDocs are being allocated just in case -ddump-*-trace is
enabled. This is awful.

We avoid this by ensuring that the dump flag check is inlined into the
call site, ensuring that the tracing document needn't be allocated
unless it's actually needed.

See Note [INLINE conditional tracing utilities] for details.

Fixes #18168.