Guard against unnecessary array detaches in attribute creation

[nvmkuruc]

Description of Change(s)

Consider the following snippet--

# testDetach.py
from pxr import Usd
from pxr import UsdGeom

stage = Usd.Stage.CreateInMemory()
points = UsdGeom.Points.Define(stage, "/points")
points.CreatePointsAttr()

Running

VT_LOG_STACK_ON_ARRAY_DETACH_COPY=1 python3 testDetach.py

produces 16 VtArray detaches due to non-const access of override property names in schemaRegistry.cpp. While not verified, we suspect that there are two detaches per schema in its type hierarchy (Points has 7 ancestors plus itself => 8 total schema types).

This guards against detaches by both making the VtTokenArrays const. Rerunning testDetach.py with this change and VT_LOG_STACK_ON_ARRAY_DETACH_COPY=1 shows zero detaches.

Fixes Issue(s)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[nvmkuruc]

Thanks to @andrewkaufman for discovering this issue.

[jesschimein]

Filed as internal issue #USD-8959

[spiffmon]

Wow, nice find, @andrewkaufman !