[llvm][mustache] Add support for Triple Mustache (#159183)
We extend the logic in tokenize() to treat the `{{{}}}` delimiters
to treat it like other unescaped HTML. We do this by updating the
tokenizer to treat the new tokes the same way we do for the `{{&variable}}`
syntax, which avoid the need to change the parser.
We also update the llvm-test-mustache-spec tool to no longer mark Triple
Mustache as XFAIL.
