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

[Alexis Engelke via cfe-commits]

aengelke wrote:

As an alternative to the exception+no-RTTI for the clang unit test, we could also consider to only build the test if `LLVM_ENABLE_EH` is set and get rid of the per-target overrides entirely.

cc @vgvassilev 

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