Export LLVM_ENABLE_RTTI and LLVM_ENABLE_EH in LLVMConfig.cmake so
clients of LLVM know if RTTI and/or EH were enabled in the build of
LLVM they are trying to link against.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213664 91177308-0d34-0410-b5e6-96231b3b80d8