[CSSPGO] Skip reporting staleness metrics for imported functions

Accumulating the staleness metrics from per-link is less accurate than doing it from post-link time(assuming we use the offline profile mismatch as baseline), the reason is that there are some duplicated reports for the same functions, for example, one template function could be included in multiple TUs, but in post thin link time, only one function are kept(linkonce_odr) and others are marked as available-externally function. Hence, this change skips reporting the metrics for imported functions(available-externally).

I saw the post-link number is now very close to the offline number(dump the mismatched functions and count the metrics offline based on the entire profile), sightly smaller than offline number due to some missing inlined functions.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D156725