[clang-tidy] refactor ExceptionAnalyzer further to give ternary answer

Summary:
The analsis on the throwing behvaiour on functions and statements gave only
a binary answer whether an exception could occur and if yes which types are
thrown.
This refactoring allows keeping track if there is a unknown factor, because the
code calls to some functions with unavailable source code with no `noexcept`
information.
This 'potential Unknown' information is propagated properly and can be queried
separately.

Reviewers: lebedev.ri, aaron.ballman, baloghadamsoftware, alexfh

Reviewed By: lebedev.ri, baloghadamsoftware

Subscribers: xazax.hun, rnkovacs, a.sidorin, Szelethus, donat.nagy, dkrupp, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57883

llvm-svn: 354517