[llvm] Make llvm::Any similar to std::any

This facilitates replacing llvm::Any with std::any.
- Deprecate any_isa in favor of using any_cast(Any*) and checking for
  nullptr because C++17 has no any_isa.
- Remove the assert from any_cast(Any*), so it returns nullptr if the
  type is not correct. This aligns it with std::any_cast(any*).

Use any_cast(Any*) throughout LLVM instead of checks with any_isa.

This is the first part outlined in
https://discourse.llvm.org/t/rfc-switching-from-llvm-any-to-std-any/67176

Differential Revision: https://reviews.llvm.org/D139973