[NFC] ExceptionEscapeCheck: small refactoring

Summary:
D59466 wants to analyse the `Stmt`, and `ExceptionEscapeCheck` does not
have that as a possible entry point.
This simplifies addition of `Stmt` analysis entry point.

Reviewers: baloghadamsoftware, JonasToth, gribozavr

Reviewed By: gribozavr

Subscribers: rnkovacs, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D59650

llvm-svn: 356799