clang/Modules: Sink ASTReadResult in ReadControlBlock, NFC

Simplify the code by avoiding some state that wasn't being used.  The
function-level `Result` was only assigned a value other than `Success`
in the handler for `OPTIONS_BLOCK_ID`, but in that case it also hits an
early return.  Remove it at the function-level to make it obvious that
the normal case always returns `Success`.