[BOLT] Fix merge-fdata handling of BAT profiles

When a profile is collected in a BOLTed binary, the generated
profile is tagged with a header string "boltedcollection" in the first
line of the fdata file. Fix merge-fdata to recognize this header
string and preserve it into the output.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D125591