[demangler][NFC] Tweak legacy uuidof handling

We have to special-case 'u 8__uuidof [tz]' demangling for legacy
support.  That handling is a little duplicative.

* It seems better to just push the single expected node.

* We can also use 'consumeIf' rather than open-coding the peeking and increment.

* We don't need the numLeft < 2 check, as if there are few than that
  other paths will end up with detecting the error.

FWIW This simplifies a future change adding operator precedence.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119543