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.

llvm-svn: 213663