swift234

Move the "type conforms to protocol" cache into the ASTContext arena.

This fixes an awful nondeterministic memory smasher involving cases
where the type checker checks whether a type involving type variables
conforms to a given protocol. The checks were cached in an
ASTContext-level data structure, but the keys involved
constraint-checker-allocated types. Stale entries in the cache caused
all manners of unreproducible weirdness, almost surely including
<rdar://problem/15715339>, <rdar://problem/15736793>,
<rdar://problem/15768325>, and probably others.

Swift SVN r12898