[llvm] [llvm-profgen][NFC] Detect pre-aggregated format (PR #191593)

[Amir Ayupov via llvm-commits]

aaupov wrote:

> Why do we need to restrict buildID annotation only for LBRStack samples? 

We can add buildid prefix support in two ways:
1. narrow: pre-aggregated only where it's needed,
2. uniform: pre-aggregated and perf script input where it's not needed.

I chose option 1 to keep perf script parsing strict.

https://github.com/llvm/llvm-project/pull/191593