Handle standard library functions in the analysis engine

[guodongli-google]

Bug report

Describe the bug
Go has a large number of library functions in the standard package, whose source code may be missing and the propagation analysis may have no clue on their semantics.
While we can hard-code the support for some known ones, e.g. fmt.Printf(...), it may be impractical to do that for all the functions. If we over-approximate their semantics, then false positives will be generated.
This bug is to track the progress of the support for these library functions.

[mlevesquedion]

I see you have already added a hook visitKnownFunction for this purpose.

FWIW, the non-EAR version of the analysis already supports this to a large extent. See the internal/pkg/propagation/summary package.

Perhaps these summaries can be used by EAR as well.