Add basic parser support for operands:

 - This introduces a new FunctionParser base class to handle logic common
   between the kinds of functions we have, e.g. ssa operand/def parsing.
 - This introduces a basic symbol table (without support for forward
   references!) and links defs and uses.
 - CFG functions now parse and build operand lists for operations.  The printer
   isn't set up for them yet tho.

PiperOrigin-RevId: 205246110