[clang-tidy] Small refactor for ExceptionAnalyzer

- Use llvm::DenseMap<> with pre-allocation instead of std::map<> for FunctionCache
- Avoid double lookup for FunctionCache
- Use try_emplace instead of insert
- Simplify definition of State enum

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D147376