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

[Nikita Popov via cfe-commits]

https://github.com/nikic commented:

Given that any change to cmake seems to break something, can you please split these changes further? I'd like to have one change to drop `-DDGTEST_HAS_RTTI=0` (which seems fine), one to change the exports (which also seems fine) and one for the rest, which is more involved.

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