PGO: instrumentation based profiling sets function attributes.

We collect a maximal function count among all functions in the pgo data file.
For functions that are hot, we set its InlineHint attribute. For functions that
are cold, we set its Cold attribute.

We currently treat functions with >= 30% of the maximal function count as hot
and functions with <= 1% of the maximal function count are treated as cold.
These two numbers are from preliminary tuning on SPEC.

This commit should not affect non-PGO builds and should boost performance on
instrumentation based PGO.