Tracer scopes are used in numerous places in src/heap to track time
spent during various phases of the garbage collection. Usually, they
are introduced with the TRACE_GC* family of macros, taking the scope
identifier as a parameter. At most call sites, the scope identifier is
known at compile time.

This CL inlines the constructor and destructor of GCTracer::Scope, in
order to enable the C++ compiler to properly optimize the introduction
of such scopes when the scope identifier is known at compile time,
using constant propagation. This is expected to have a performance
impact for short-lived and frequently used scopes, e.g., in incremental
marking and sweeping steps.

Change-Id: I6f1a2954a437de1fa6dab5e464c20c952d84ffd4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3581774Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79957}