FieldRef print method (operator<< llvm::raw_ostream?)

For debugging (`-debug` but also adhoc print/dumps) purposes, add a printing helper for neatly formatting FieldRef's.

Presently they appear to be handled by implicit cast to bool (printing 1/0), which is not as useful as it could be!
Either directly print some inline representation of the value with the specified FieldID or consider a friendlier approach re:`getFieldName` or similar (at cost of losing, e.g., the op/SSA value directly?).

No file or test is named. Start by locating the FieldRef definition and inspecting how it is currently printed or implicitly converted during debug output. The work is done when FieldRef has a useful, agreed-upon representation that preserves the specified FieldID or relevant field information instead of printing only 1/0.

Written by the indexing model from the issue text.