[ThinLTO] parse flags and blockcount summaries

Forked from pr/46523, we were having a hard time running llvm-extract on
IR from a thinLTO build of the Linux kernel.

$ llvm-extract --func jeq_imm jit-42f488b6.ll
llvm-extract: jit-42f488b6.ll:47463:8:
error: Expected 'gv', 'module', or 'typeid' at the start of summary
entry
^209 = flags: 8
       ^

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D82917