[llvm] Build llvm with rtti enabled.

Recent llvm versions use llvm::Error as a facility for error handling. It
includes an ErrorInfoBase class which has virtual methods (with a pinned vtable
in the object file). This makes it harder to include headers that use
transitively include llvm/Support/Error.h crossing rtti/no-rtti worlds (that is
llvm is compiled with -fno-rtti and TCling and friends with -frtti).

Newer llvm versions have a lot of usage of llvm::Error in its header files and
it is practically impossible to solve this problem by avoiding the headers
that use ErrorInfoBase.

There are two other alternatives. First we can outline the functions in Error.h
which use ErrorInfoBase but then we will hit other things like RTTIRoot which
also needs special treatment. Secondly, we can enable llvm's RTTI builds for
ROOT and this is what this patch does.

interpreter/CMakeLists.txt

@@ -4,6 +4,7 @@ if (NOT builtin_clang)
 endif()
 
 #---Define the way we want to build and what of llvm/clang/cling------------------------------------
+set(LLVM_ENABLE_RTTI ON CACHE BOOL "")
 set(LLVM_APPEND_VC_REV OFF CACHE BOOL "")
 set(LLVM_ENABLE_BINDINGS OFF CACHE BOOL "")
 set(LLVM_ENABLE_FFI OFF CACHE BOOL "")