swift234

Introduce a special memory arena in ASTContext for the type checker.

This introduces the notion of arenas into ASTContext, with two arenas
currently defined: one for 'permanent' storage, and another for the
current constraint checker. The latter is used when allocating any
types that involve type variables, which are only used temporarily
during type checking anyway.

This gives us a 1% speedup on swift.swift (because we're hitting
smaller hash tables when doing lookups) and < 1% memory reduction
(since that's not the main source of memory usage). It's more
important architecturally, so our memory usage doesn't grow with the
number of type-checks performed.

Note also that this arena scheme could be generalized, which we may
very well want to do in the future. For example, we could easily have
an arena for temporary nodes introduced by parsing (e.g.,
UnresolvedDeclRefExpr) or by name binding (OverloadedDeclRefExpr), and
clear that arena when we successfully move onto the next phase. Or, in
a REPL/debugger context, have a 'temporary' arena for
statements/expressions that can be removed.



Swift SVN r3175