Avoid conversion of SdfLayerRefPtr to SdfLayerHandle in pcp/cache.cpp

[nvmkuruc]

Description of Change(s)

The following warning was observed in GCC 11.

warning: loop variable ‘layer’ of type ‘const SdfLayerHandle&’ {aka ‘const pxrInternal_v0_23__pxrReserved__::TfWeakPtr<pxrInternal_v0_23__pxrReserved__::SdfLayer>&’} binds to a temporary constructed from type ‘const pxrInternal_v0_23__pxrReserved__::TfRefPtr<pxrInternal_v0_23__pxrReserved__::SdfLayer>’ [-Wrange-loop-construct]
 1232 |         for (const SdfLayerHandle& layer: layerStack->GetLayers()) {

GetLayers() returns a vector of SdfLayerRefPtrs. Iterating using a handle will trigger an implicit conversion to a temporary SdfLayerHandle. This PR uses auto to preserve the type of the vector contents and avoid any implicit conversions.

Fixes Issue(s)

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

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8647