Added LLVM_ENABLE_RTTI and LLVM_ENABLE_EH options that allow RTTI and EH
to globally be controlled. Individual targets (e.g.  ExceptionDemo) can
still override this by using LLVM_REQUIRE_RTTI and LLVM_REQUIRE_EH if
they need to be compiled with RTTI or exception handling respectively.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213663 91177308-0d34-0410-b5e6-96231b3b80d8