[llvm-profgen] Deduplicate PID when processing perf input

When parsing mmap to retrieve PID, deduplicate them before passing PID list to perf script. Perf script would error out when there's duplicated PID in the input, however raw perf data may main duplicated PID for large binary where more than one mmap is needed to load executable segment.

Differential Revision: https://reviews.llvm.org/D111384