[analyzer] Fix StdLibraryFunctionsChecker performance issue

`initFunctionSummaries` lazily initializes a data structure with
function summaries for standard library functions.  It is called for
every pre-, post-, and eval-call events, i.e. 3 times for each call on
the path.  If the initialization doesn't find any standard library
functions in the translation unit, it will get re-tried (with the same
effect) many times even for small translation units.

For projects not using standard libraries, the speed-up can reach 50%
after this patch.

Differential Revision: https://reviews.llvm.org/D98244