[clang] [analyzer] Allow SVals as llvm::Immutable{Map, Set} keys (PR #205319)

[Balázs Benics via cfe-commits]

steakhal wrote:

This trait was inspired by https://github.com/llvm/llvm-project/blob/main/clang/include/clang/StaticAnalyzer/Core/PathSensitive/SymbolManager.h#L696-L721

https://github.com/llvm/llvm-project/pull/205319