[demangler][NFC] Tabularize operator name parsing

We need to parse operator names in 3 places -- expressions, names &
fold expressions.  Currently we have 3 separate pieces to do this, and a FIXME.

The operator name and expression parsing are implemented as
handwritten two-character nested switches, the fold expression is a
sequence of string comparisons.

This adds a new OperatorInfo class to encode the operator info
(encoding, kind, name), and has a table that it can binary search.
From that each of the above 3 uses are altered to use the new scheme.

Existing tests cover parsing operator encodings.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119467