[demangler] Add operator precedence

The demangler had no concept of operator precendence, and would
parenthesize many more subexpressions than necessary.  In particular
it would parenthesize primary-expressions, such as '4', which just
looks strange.  It would also parenthesize '>' expressions, just in
case they were inside a template parameter list.

This patch fixes both issues.

* Add operator precedence to the OpInfo structure, and add a
  subexpression helper that will parenthesize a lower precedence
  subexpression.

* Add a 'greater-than is greater-than' indicator to the output buffer,
  so the expression printer knows whether it is immediately inside a
  template parameter list (and must therefore parenthesize 'expr >
  expr').  This is a counter, so that ...

* Add open and close printers to the output buffer, that increment and
  decrement the gt-is-gt indicator.

* Parenthesize comma operators inside comma-separated lists. (probably
  a rare case, but still).

This dramatically reduces the extraneous parentheses being printed.

Reviewed By: dblaikie, bruno

Differential Revision: https://reviews.llvm.org/D120905

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