[llvm-profgen] Deduplicate and improve warning for truncated context

This change improves the warning for truncated context by: 1) deduplicate them as one call without probe can appear in many different context leading to duplicated warnings , 2) rephrase the message to make it easier to understand. The term "untracked frame" can be confusing.

Differential Revision: https://reviews.llvm.org/D109115