[clang] [flang] [llvm] [CMake] Drop per-target LLVM_REQUIRES_RTTI (PR #174201)

[Alexis Engelke via cfe-commits]

aengelke wrote:

It'd also be great if someone could try this with MSVC, the pre-commit CI uses clang-cl. (Alternatively, merge + revert on failure?)

https://github.com/llvm/llvm-project/pull/174201