consider enabling some KHR_ICD_TRACE messages by default

One of our users was recently debugging a tricky library dependency issue that was preventing one of our OpenCL devices from enumerating. Enabling KHR_ICD_TRACE messages (via the OCL_ICD_ENABLE_TRACE environment variable) was a huge help debugging the issue, but most users (especially those who use OpenCL via higher-level language or libraries) won't know to set this environment variable. To help debug similar issues in the future does it make sense to enable some KHR_ICD_TRACE messages by default, say for exceptional conditions that are preventing OpenCL or OpenCL devices from functioning?

I can put together a more specific proposal but I wanted feedback on the overall concept first. Thanks!

Start by reviewing the loader's current KHR_ICD_TRACE handling and how the OCL_ICD_ENABLE_TRACE environment variable enables messages. The issue does not name files, tests, or a defined set of exceptional conditions, so first turn the concept into a specific proposal with clear criteria for which messages should be enabled by default.

Written by the indexing model from the issue text.