cc: extract helper methods from monster functions

Extract builtin dispatch from parse_primary_expr (1544→250 lines)
into parse_builtin_expr. Extract four groups from linearize_expr
(920→247 lines): linearize_compound_literal, linearize_va_op,
linearize_builtin, and linearize_c11_atomic. Pure mechanical
refactor, no logic changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jgarzik