By using different namespace, generated code split into different files.

By this CL, build time of torque_generated_initializers is improved from 80 seconds to 40 seconds on Z840 Linux measured by following command:

# Remove generated code to re-compile all generated files.
~/v8/v8$ rm -rf out/x64.release/gen/
# GOMA_STORE_ONLY=true disables goma's backend cache.
~/v8/v8$ time GOMA_STORE_ONLY=true autoninja -C out/x64.release/ torque_generated_initializers

Bug: v8:8732
Change-Id: I64545f9994aea7e4209ac3852aadf4e2e9f4bc93
Reviewed-on: https://chromium-review.googlesource.com/c/1446331Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59215}