[analyzer] StdLibraryFunctionsChecker: Associate summaries to Functio…

…nDecls

Summary:
Currently we map function summaries to names (i.e. strings). We can
associate more summaries with different signatures to one name, this way
we support overloading. During a call event we check whether the
signature of the summary matches the signature of the callee and we
apply the summary only in that case.

In this patch we change this mapping to associate a summary to a
FunctionDecl. We do lookup operations when the summary map is
initialized. We lookup the given name and we match the signature of the
given summary against the lookup results. If the summary matches the
FunctionDecl (got from the lookup result) then we add that to the
summary map. During a call event we compare FunctionDecl pointers.
Advantages of this new refactor:
- Cleaner mapping and structure for the checker.
- Possibly way more efficient handling of call events.
- A summary is added only if that is relevant for the given TU.
- We can get the concrete FunctionDecl by the time when we create the
  summary, this opens up possibilities of further sanity checks
  regarding the summary cases and argument constraints.
- Opens up to future work when we'd like to store summaries from IR to a
  FunctionDecl (or from the Attributor results of the given
  FunctionDecl).

Note, we cannot support old C functions without prototypes.

Reviewers: NoQ, Szelethus, balazske, jdoerfert, sstefan1, uenoku

Subscribers: whisperity, xazax.hun, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, donat.nagy, dkrupp, gamesh411, Charusso, steakhal, uenoku, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77641